If my friend and I create an alliance, and form 3 player games so that we can team up on the third guy, is it okay as long as we announce that we are teaming up on him?
Ummmm i dont think that would be a wise decision. That does not sound like correct alliances. Alliances usually are formed for when a player is in the lead. This sounds more like major abuse to get points.
If you have already formed the alliance before you start the game, then it's secret diplomacy and against the rules.
Even if you don't say anywhere that you have an alliance, if you play in such a way that you deliberately team up on the 3rd player in 3 player games, it is considered to be against the rules.
If you want to team up with your friend, play doubles.
If you're not trying to win the game with your alliances then its going to be viewed as cheating. Remember that throwing games is also against the rules so if you make an alliance with the most powerful player and basically give him the game (even if that most powerful player happens to be your friend), that could also get you in trouble.