To output a blue belt (40/s) of GC, this setup requires less than a blue belt of iron and copper respectively.
The extra fast inserters supplying iron and copper only serves the purpose of smoother production WHILE those placing GC are necessary because 3 fully upgraded stack inserter together cannot transfer 40 items/s onto the belts.
Theoretically these 3 GC assemblers are capable of producing more than 40GC/s, the only limitation comes from the speed of 1 blue belt.
The gaps between beacons are intentional so as to support each assembler with 8 beacons.
Edit: added missing electric poles and made minor adjustment.