I freaking love greys! One of the big reasons I like manga so much. I like what solhawthorne said: I think more contrast is necessary. To me that means you'll need to add some more grays but also make use of blacks and whites more. So, for instance, use your three grays, but use them sparingly. Then add more black and white. Although, this will give a very different look than you currently have.
It might be a good idea to look at black and white comics that are just made in ink. Everything is black and white and for a "grey" crosshatching is commonly used. In your case, grey would be used instead of cross hatching.