My personal suggestion:
1) Let them set the pace (as stated before).
2) Also, let them KNOW...up front...that you are not there for sex. You are there to strengthen the bond with them as a relationship. (assuming you are trying to become part of their relationship dynamic and not just in for sex)
3) I also like the "be easy on my guys...I'm new to this" approach. It lets them know to set the pace and go a little slower. If you feel like it's going too quickly for your comfort, you can say "woah...lets hold off on that till a little later. Ok guys? I'm just not ready for that yet.". The will likely have an appreciation for that.
4) When flirting, don't be afraid to flirt with the woman in FRONT of her husband. The reason? If you wait till he leaves the room/area, he may feel you are trying to get her alone to cheat with you on him. (My wife felt this recently when our gf would do that with me)
5) Find out what's acceptable early on. A simple phrase and question, can help to eliminate possible problems; "Hey...while we're on the relationship subject, what do you guys feel is acceptable in the beginning of a relationship with someone?" I have heard of instances where one party or the other was not comfortable with _______ or _______" . Insert whatever you may feel might be a problematic area. Possibly holding hands, or kissing, or if it progresses to a sexual relationship early on - anal sex or oral sex or ???
5) Try to keep it simple. Ask them what they are looking for in the relationship. A boy toy? Or a lover? or a second husband? or what?
Now, these are just MY opinions. As a husband in a married poly couple, who has had a second male involved. But we are not in that dynamic anymore. I know that I had moments when I got upset. I got nervous when my wife's BF asked if he could take her on a date while I was at work. I was very against that for some weird reason. It made me upset. So make sure you approach things gingerly.
Hope we have helped.