I agree with Jason. (Excellent job bud.) Hard to believe it or not, jealousy(envy) between guys/bros occurs just as it does between girlfriends/sisters. I myself have been in similar situations such as the one you described; twice in your position, and once in your best friend's position. How I solved my situation (while being in your role the 2nd time) was basically just acccepting/getting to know the third friend. Maybe he's not that bad of a guy to be around with? (Who knows? You could someday prefer him over your best friend.)----But, If you truely don't like/don't want to be around with the new person/he's against you more than Reject is with the movie
Crash /or its just not working out~And its truely eating you~, well...that's when it gets and feels awkward. And the only reasoned way I've found (In your similar situation the first time) is to immediately deal with that weird feeling by just talking w/ your best friend about the problem.(But please, not in public.) How it'll turn out in your case? I sure don't know, but its a great test of a friendship. When I took that approach, it was a better choice than leaving the feeling bottled up.~ Because if I left that feeling in an empty patronizing bottle of Budweiser, I would've had a terrible time w/ my friends, and it would've gotten in the way of all the great/fun times we had together later on in life. (And all the stuff we now have planned in about 2 weeks

, when college is back in sessi

n. ) Basically, that grudge would've been more negatively ahold of me than the one Sarah Michelle Geller is going to get in her next movie. (Bad, ironic, barely-helpful, jovial pun? I know.

) Having that feeling (and yes certain women along with certain gay men, we do have them.) probably would've killed me in more ways than one.
Anyways, I have nothing more to add onto all this.
Keep Cool.