Oh wow I didn't know this kind of program exists! I'll check it out, thanks 
Oh, well i'm sorry for that, i assumed you didn't know how to draw backgrounds. But it still applies, practicing would help you do it faster.
If you really want to make 3D backgrounds though, I'd recommend Blender. It's a little harder to learn than sketchup, but it's still fairly easy compared to other similiar programs. I't also not just for rooms and buildings, it lets you model furniture and decorations and all of that stuff.
Thats okay its my fault for not saying it in the post lol. And yeah, I'll practice some more in my spare time. I just googled Blender, it looks difficult but wow it sure can do lotsa stuff 
I only know SketchUp so its nice to know that there are other programs that can do similar things! Thanks for telling me 
If you want to use/trace existing 3D models in your backgrounds, like anything else, you'll have to look at the terms of use the creator of said model has set up.
I occasionally put together scenes in SketchUp using whatever models I need (usually some of my own and some free models) and use that as reference for my drawings. I don't really recommend reinventing the wheel by making all your SketchUp models from scratch. Unnecessary time sink, unless you're also looking to build a 3D modeling portfolio.
Do you need actual models though? Sometimes if I need a quick room mock up, I'll open Maya (free alternative would be Blender--Sketchup will work too, and is not hard to learn, but it can be a little buggy and finicky) and whip up some primitives (basic shapes like globes, cubes, etc) to vaguely resemble a room. Infinitely quicker than setting up a room in SketchUp. You can freely trace that if you like and add curves and details as you see fit. This allows for and encourages more creativity.
Blender is great and all and I do recommend it as well, just wanted to point out that making furniture and decorations in SketchUp is actually pretty easy and part of what it is meant to do. Hence why the SketchUp Asset store is 90% furniture, appliances and decor.
Sketch up is actually fantastic. I have used it before to create some 3D maps for a roleplaying game supplement. The idea was that the players could open the map as a software file and move around the tiny town I sketched and see which points have line of sight on which other points.
Is it easy to use? Pretty much unless you are a perfectionist. It can get a little glitchy, so I wouldn't recommend it in place of real modeling software for anything like blueprints. It has fun textures and very easily can make crude 3D objects. I could see a draftsperson using Sketch Up to make a rough draft of a house for a presentation within minutes and then doing it for real using pro software.
Back to the question about using models in comics, I did skim the Terms of Use. The answer is -- maybe?
First, we have this text.
Currently all Models are made available and Distributed through 3D Warehouse free of charge. If Trimble implements a payment system, Trimble will post additional terms to these Terms of Use describing the ways in which Models may be purchased and sold through 3D Warehouse.
Later, we have that other text.
You may not:
Modify 3D Warehouse Materials (other than your User Communications or Models or content you posted) or use them for any public display, performance, sale, rental or for any commercial purpose except as expressly authorized in these Terms of Use or the General Model License;
If it were me, I would credit the creator of the specific model that is being used.
I say just create the background in sketch up(your own original one), put the camera how you want it and then just trace the environment, seen too many comics using 3d models for 2d characters and it looks weird too me, it's easy to spot and takes me out of it
@GalacticPotato
as someone who has practice perspective a lot, it dose get easier just not quicker XD
Seconding Idiotwithapencil. It's a good tool but only when it's used as a tool and not a crutch. Don't use filters and just slap it into your comic because it looks incredibly jarring against hand drawn characters. It's really off-putting reading comics that do this. The most I've ever used it for is to just set up some basic horizon lines, take a simple rectangle outline and build my own environment from there.
if you wanna get up your webcomic production to official webtoons speed (40 panels a week) and get a better shot of being pro, better learn sketch up or get a partner who is a background specialist with prociency in sketch up.
the pros do it, why shouldnt you?
you dont have to be good at everything.
oh yeah trace the backgrounds obviously, mixed media is still far away from being acceptable.
I agree with what Spudfuzz and WednesdayAsh said.
Sketchup allows you to use the models for your comic, as long as you don't use it for commercial use (unless you purchased a license).
Aditionally, I've made 3D models before (although I don't post them online, since it's simply not my focus), so I'd be flattered if they were used in someone's comic to be honest.
And pros use Sketchup all the time. Yuumei169, Tsulala109, Feigap95, and many more!
If you learn how to use the program, it's a skill and not a crutch.
A lot of anime artists also use 3D programs to draw backgrounds, so use any programs available to your advantage. I'm not a big fan of those who don't trace the backgrounds (they just take the model and put it in their comic) because things just feel out of place. Of course, that's just my preference and some comics can make it work better than others, such as Killing Stalking.
However, I would also recommend learning perspective, since it will also be helpful to you when you want to add extra details to your buildings and such.
Or maybe you want to slightly exaggerate some buildings proportions just to match the atmosphere or a certain scene and such. So learning both would be good. \o/
hell yea go for it. if it saves you time then do it.
ive been thinking of building my own things using sketchup but the program has changed so much over the years i remember using it back in 2016 and it was so difficult to use. maybe ill use blender next time.
I'm not really into making a 3D modeling portfolio like what you said, so definitely not 
Also I wonder if it's okay to use a free model (like the interior of a room) then tinker it a bit here and there and claim it as your own ...? And I definitely do need some models, its great that SketchUp has a lot of furnitures. Since the both of you recommended Blender, I'll make sure to learn about that, same with Maya.
You make games? thats cool
I'm a bit of a perfectionist, so definitely it'll take time. Welp, time to go to youtube for tutorials 
Uwah, I didn't really look through the Terms of Use so thanks for reading it for me, haha. I guess I could just trace the model and make it look a little bit different then credit the creator or... just seriously learn how to use SketchUp so I don't need to use existing models.
That is so true! I've seen some comics like that and looking like it was out of place but its just that there are also other ones that looks like it fits just right too (webtoons like untouchable, noblesse etc.) But yeah, I'm gonna go for tracing instead just in case -.-
Yep, definitely gonna learn it. And tracing is a really good strategy 'cause I could just add things i want and it wouldn't look out of place if it was with 3D. But thats kinda sad though that mix media still isnt accepted.
i don't know what program what this artist uses but she makes really good 3D backgrounds for her COMIC253. i've seen rough sketches but never the process
Thanks for the encouragement on using SketchUp and ooooh fellow Killing Stalking reader, nice to meet you 
And yes, I know how to draw in perspective (learned it from school) though its not exactly my forte. I'm also into exaggerating drawings like foreshortening (but have yet to learn 3rd point perspective tho)
Can you give me a link of where you've seen her rough sketches?