You know. my tip. a sure win game if your fast at building and earning hold. make 2 sets of gryphons. thats an easy win. footmen and riflemen for earlygame. and 2sets of gryphons at the end of the game
not a good idea if your enemy is undead (crypt fiend type) or Orc (raiders type)
They just ensnare you gryphons helplessly
Each race has it's own set of pros and cons. And to answer the original question, always have your other 2 teammates discuss between yourselves before any game. One of you should keep dishing out the tier 1 melee fodder eg. Footmen, another should mass ranged, and another with casters.
All 3 of you should have heroes to buff all 3 sets of units (2 each is ideal). It's just a basic, cookie cutter build between 3 people. Saves alot of trouble from the micro, and the casters' owner can get his/her job done alot easier.
You want to take out miners, workers ect. as fast as possible. Riflemen are a great plus in early game although as what was stated all races have their pros and cons so it really is up to you what you want to use to fight and what works best for you. One of the best methods is getting a friend and testing out the races against each other to see which race you like best as which race you pick and how you work with it makes a big difference in the game. Another great thing to use as someone already stated is hero's 1-3 each to buff up all unit sets and allow jobs to get done much more easily.
There is no bona fida best 3v3 set up or single race for that matter. Humans require plenty of micro management in things, Undead have pitiful regeneration outside of their bases, Orcs have less anti air than other races. It all depends on how well your team can communicate and how you set up your units and such. Strategy and synergy helps out in the end.