Avoid having sex with an exes friend, especially their "best, best bestest" friend. It won't end well, no matter how hot she is.
Checking with the ex is a little tricky. You might destroy her friendship with the best friend. (Personally, she doesn't seem like a very good friend, but that is just my opinion.) If you really want to know, tell the best friend to get your exes "permission" and that you want to hear it from her (the ex). If she doesn't call, you know its a "no" (either because the best friend never asked, or the ex isn't cool with it). That way you get your answer, and it keeps you from having to broach the "Can I fuck your best friend" topic with an ex you seemed to care about.
it's a trap! doesn't matter if it is though, if you want to do it, then do it. your ex should not have anything to do with your decision. she's not your friend, she's your ex. edit: i know i'm too late on this and you've probably already done what you're going to do anyway. there's just a lack of current threads to post in right now...