ASG is very fact-intensive and there are no longer any regs, even proposed, so hard to opine. You also need to know family and business relationships for stock attribution. Having said that, it seems to me that, depending on additional facts, Firm 1 could be performing historical employee services for 2 and 3, and the owners of 1 or HCEs of 2 and 3. 1 and 2 would be one m2b, 1 and 3 the other, and you might have a "combined group." But again, this is just a hypothetical analysis of a hypothetical situation, Would need more facts. The Code section is in effect, but again no regs.
Good luck, Below Ground.