1. I don't think this one's possible. I think it has something to do with that the two squares are diagonally from each other in the corners of the box. This makes each edge contain 7 squares instead of 8. 7 is an odd number, which is not divisible by the dominoes we're trying to fill the space with. When I try to fill the box, I always end up with two empty squares that are diagonal from each other.

2. This one can be done!

3. Finally solved this one!

*We know the leftover square has to be green or orange (because there's 22 vs. 21), but none of them match up with each other! Therefore, I think it's impossible

Also, I drew a cat:)