Thanks for the help. I can't believe it took me that long to do it. I'd already tried so many things with that catapault. I'd even gotten it, and the box from a few rooms back, to try and weight all three switches at once.
Edit: I'm now stuck just after that puzzle. I'm in the room with 3 catapults, there is a platform that raises, on catapult is on it, another is on a ledge, there are levers to reset the catapults. Just can't figure out what to do here. Haven't had much trouble with the puzzles before these two