Maybe it is not a big deal but my best advice is thinking about a conventional couple and do it all in the same way but with three persons instead two. Obviously, all relationships are different, polyamory one even more I guess... but I have some friends in one and they actually looks really normal, in all senses. The most noticeable is the great confidence that they three have with each other. I do admire them a lot for that.
So... I think that reflecting this normality could be a good point. At least, that is what I usually try to do when a write/draw about this kind of themes.