Would formation of the three planes benefit from increased distance of the ground effect? I know that the ground effect quickly diminishes as altitude above the ground increases, and it is roughly half of the wing span. Hence it can be used to fly more economically over water, but over ground normally would not be high enough.

I have envisioned two larger airplanes "helping" the smaller one in between by flying next to it as shown in the diagram. By idea, wings of the all three planes would form much longer "superwing" with the ground effect equal to the complete wingspan of the larger plane plus half of the wingspan of the smaller plane in center. This would be much higher than the small plane could do alone (half of its wingspan), maybe making possible to use the ground effect also over ground. The small airplane would then need less effort to fly, if it is, for instance, low on fuel.

Of course, the obvious problem is that the wings cannot "join", it must be some reasonable gap there to avoid the collision. 

Would the central plane flying in formation as shown in the schema experience a stronger ground effect than flying alone?
I would think not; the ground effect comes from the wing pushing air down, which would not help the plane in the middle. If anything, I think the plane in the middle would be impacted by the wingtip vortices created by the larger planes, once out of ground effect (one wingspan above the ground) if the smaller plane did not stay ahead of the larger planes. 
http://www.pilotfriend.com/training/flight_training/aero/wng_vort.htm 

A position slightly behind, drafting like birds in flight do when they fly in V formation, might help. 
https://www/ciencemag.org/news/2014/01/why-birds-fly-v-formation