Sounds like everyone is cool with sloppy seconds.
What are your thoughts about having sex with a person that your friend has also slept with in the past? My friend is ok with it and he has moved on from her (he's actually the one hooking her and I up.) She's down with it too
More drama the better right?
If you have to ask ... You shouldn't be in that type of relationship, because you most likely aren't mature enough to take responsibility for the possible consequences.
Millions and millions are OK with having sex with each others  mates and ex-mates. That being said, if all of you are in  agreement with having sex with each other that's your right.Â
Considering there are no emotional obligations bewtween the ex's there is no reason to be concerned about "hooking up" .. If the original lovers did not cultivate a relationship because they were nota match why would "friends" when have a prob with it ? Having sex with someone does not "Mark" them as territory.
Not everyone has one relationship for life.
Sexual intimacy is not an unusual occurrence in today's culture. (Probably no more frequent now than in previous eras, but with more overt "acceptance.")
I had a very minimal number (low single digits) when I was single over 38 years ago. I have warm feelings for those women---although I very seldom think about them---and I hope their lives have turned out very well.
As long as the 3 of you don't currently hang around together. It's entirely up to you. Otherwise, the more frequently you are together, the more awkward it may become.
I think you are playing with fire. I am sure there are many other girls out there!