all i can say is: be honest with both of them. ive been in a similar situation- only i was part of the couple. and what hurt the most was all the stuff that went on behind my back..and then later finding out about it.
you say you dont want to ruin the friendships. you will! you cant have it all.
someone will get hurt so make sure its really worth it.. before acting on an impulse that may just feel "deep" now.
oh and sorry but: if hes flirting with you now and has a girlfriend at the same time- well im sure there is some saying about that..about some people never changing their ways!?
this might not be what you want to hear but it is a subject im kinda familiar with.

firstly do not try and start a relationship behind the other girls back, that is the worst thing for everyone involved

i think, like germany said, you have to decide if you truly care for this guy and if you do try talking to him, make sure he feels the same way,
but someone will get hurt in this situation, possibly for everyone involved,

this may not be much help, but i think this is one of those cases where you have to decide what your willing to risk to gain this relationship