Would formation of the three planes benefit from increased distance of the ground effect?
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