[Eternal]

Well, as you know things won't work out unless the problems that caused your relationship to fail the other two times has been fixed. In this case, it's her friend that's been causing problems. Perhaps you could try talking to your friend about it, and see if you can both agree to just not bring up her friend in conversation? If she remains out of the picture, then you can just focus on your relationship.
It sounds like you really do want to get back together with her as well, and honestly if you've lasted 3 years of having some form of contact, then I think there is a chance that it could work out. You should definitely start by taking things slow though, and if you realize that it might not work out again, you can get out of it before things get messy and you lose her. It almost sounds to me like you're already acting like you're back together, you just haven't made it official.
If you do decide to give this one last try, you should also try and just completely reset and leave the past behind. You've both changed each time that you've tried this relationship, and so past mistakes shouldn't make a difference in the present.

So to answer your question, there isn't really a way of knowing if things are right unless you give it one more try. If you don't take the risk of trying this one more time, then you'll never know if things could have worked out. As for getting jealous, I don't know the situation with your friend's friend, but she obviously wants you and not her. And the fact that she wants to try this for the third time is pretty good evidence that she really likes you and wants this to work.