I'm putting together a similar group this month, as a birthday present to myself. Three providers + me. I've done 3 somes before, always with my most regular provider. He and I started planning the evening earlier this summer by discussing whom else I might want for the birthday. I sort of gave him veto power, as I've run my choices by him. I have guy no. 2 on board (another semi-regular hire of mine) and soon to close the deal (I hope) with provider #3 (who'll be new to me, and to #1 and #2 as well).
What's different from the OP is that I've hiring for different times: #3 for 1.5 hours, #1 for 4-5, and #2 for overnight. So sometimes I'll be having a one-on-one, sometimes playing with 2, and sometimes with all 3.