I for one, would be surprised if NEF made a special receiver for the 3-G combo's. Not trying to be argumentative.
Did a comparison of the six 3-gun combo's offered with the seven 2-gun combo's also offered by NEF. I came away thinking they're using std. SB-one's and two's, and the way the product-combo's are lsited are what's "confusing."
Also, depending on how exact a receiver and barrel-fit needs to be, I could see NEF's inspectors hand-selecting which receivers might be used for that program/product.
As for SB-1's versus SB-2's, I thought DIMENSIONALLY they are the same, but that SB-2's are made of tougher heat-tempered steel able to handle the higher pressures of the full-size CF cartridges offered.