Finding a 3rd in open/poly spaces as a guy

So me and my partner are looking for a 3rd. We have had a bunch of threesomes, but usually at play parties or similar.

We have a rule where I can flirt with girls and make out alone, but anything farther needs to involve all of us. We also go to a lot of very open parties where lots of people are poly or sex positive.

My question is, as a guy flirting with girls with the intention of finding a 3rd, how far should I go before letting them know I have a partner and trying to move things to the three of us? Also, how should I segue that in a respectful way?