hehehee... aiya.. long time oredi din get myself involved in this kinda thing.. ahhaha.. so I figured I might as well just give it a shot.
Anyway, in love, everyone is being selfish. I mean, love itself is not selfish but people are being selfish. If you don't even wanna try to go for her, then forget it. You are selfless, so you let others grab her. If that is all it's about, then you can forget about her. It's just like animals. If the lion is there to grab the food and the cheetah fears the tiger and lets him take the food, then the cheetah is gonna die in no time.
My meaning is, be selfless when you have to but seize the opportunity when it comes. If she is happy with the other guy and she really loves him, and the guy loves her as well, this is probably where you might wanna swallow your feelings. However, if her bf is treating her badly, then there is no point holding back, I mean, would she be happy with her current BF even when you hold back? NO.
So you probably can voice it out to her in that case and let her decide. She can always move on to a better package if she finds you better. At least she knows someone out there cares for her.
I do not condone 3rd party person. I mean, I can understand how it feels when there is a 3rd party. I mean if our relationship has been good and a 3rd party comes and screw it up, then I'd be really upset. But if our relationship has been a sour one and a 3rd party comes along, I can only live to regret it and I can only change and hope she would stay with me than to go with the other guy. After all , our relationship has been sour all this while and I really have no reason to make her stay, do I?
My point here is, in a 3rd party relationship, it depends. Do it the right way and no one is to blame for that. Screw up a good relationship because you're selfish and want her for yourself, not caring about her feeling is just WRONG. If you wanna be fair to yourself, you can always wait till she ends her current relationship and then tell her but that probably leaves you at the disadvantage, then again, no harm just so to be safe not to offend anyone, including her. :)