Divide up the four outputs of the ZW-L the way you see power being used. If you have three mainlines and then sidings, that might be a logical way to split the four outputs. You have to divide the power based on the specifics of the layout.
I have a long folded dogbone main of 140 feet of track, that is on one PH180 transformer. I have a turntable and access loop, that is on one PH180. I have two smaller track loops that I can either run something on or park trains on, they are on a PH180. Finally, I have all my sidings in several areas, all of those share one PH180 and the individual tracks are all switched so I can power up only the siding that I want to use.
Here's what it looks like now.