I gotta say that making a whole group just for one's target population is a creative new hunting tactic. Props for originality. I should set up a "With-It Blokes Between 35-50 Who Are Also Reasonably Geeky and Open To Poly" group. What say, CdM? You're only a couple years off the target; you could join, too!
Okay, putting on my serious dating advice hat: be prepared to crash and burn hard before things work out. If you're committed to a closed triad, commit now or forever hold your peace. This means if your prospective partner can't in all honesty say zie wants to be with you both (or ends up deciding zie likes one better than the other), then you've got to be firm. I would be horribly disappointed, for example, if a pair I was seeing decided to dump me just because I didn't have chemistry with one of them--so I'd rather know up front that a couple is a package deal.
Also, it's really very nice to know more about said couple than "We want YOU!" What are your hobbies? Do you have children? Do you want more? What are your extended families like? Do your friends know about poly? And so forth. You are people looking to date people, not roles looking to date roles.