You can’t take the third catapult to go on the third tower in Ouranos Dungeon?
You have to completely finish both towers, because you need to enable 2 generators, not just one.
If you’re sure to have activated all the generator and finished both towers then go to the center of the dungeons (to go simply write this command in the chat box) :
/tp -1508 106 -1099
And write the following commands :
/setblock -1512 104 -1091 redstone_block
/setblock -1512 104 -1108 redstone_block