If you don't have the Output Transformation tab, you're running Avenue.
In Avenue, it's very simple to do as well:
Change your Comp Settings to be the size of your Trips output. Let's say that's 3072 x 768. If you have your Trips connected, Resolume will show you this as an option in the Comp Size dropdown.
Then turn on layer Width and Height for each layer. Resolume will default to the comp size, so this is where you can use '/3' to get each layer at 1024 x 768.
Then place each layer horizontally. Layer 1 should be at X Position -1024, Layer 2 can stay at 0, Layer 3 should be at X Position 1024.
Now every clip you play in each layer will scale and position to the left, middle or right, regardless of the original size or position.
Attached is a basic comp with the above settings.
If you want, you can get jiggy with extra layers that are set to scale over all three projectors, or combinations of two at a time.