The camera! Seriously, the camera in TS3 sucks when building. It cannot focus properly when you have all floors+roof active. When I colour and build my house, I really prefer having it all walls and floors up, but then the camera flies away so you can't have your house centered, because it focuses too much on the roof. If I move the camera slightly down, it falls down to the ground instead. There is no in-between.

There was no problem in TS2, so I don't know how they could fail this badly.