Assuming a 3' elevation and an 18" projection, you would want about 105 gpm for 7 feet of sheer descent.
Assuming 100' of 1.5" pvc, the velocity would be 16.56 feet per second and the head loss would be 61 feet.
The IntelliFlo can do 105 gpm at 78 feet of head, so something doesn't make sense. Maybe the pump is cavitating. Is the pump loud?
If you're using unfiltered water, the sheer might be clogged with debris.
The suction plumbing should be 3" and the discharge should be 2.5" pvc.
Can you give a detailed description of the system in all detail?
All suction plumbing. All return plumbing. Filter make and model. Sheer make and model. Sheer height. Sheer elevation, distance from pump to pool. Etc.
If you combine all suctions into one pump and use a big cartridge filter, you might be able to make this work with one pump.
If you have a single 1.5" line to all three sheers, that's a bad design and would be difficult to make work. If you have a 1.5" pvc pipe to each sheer, that's much better.
In any case, it sounds like the system was so poorly designed that it might not be possible to make it work without serious renovation.
Pictures of everything would help.