I wonder if there are some nice strategies / match-ups / build orders / resource sharing / tactics for 2x2, 3x3, 4x4 player teams? (3x3 is most interesting for me.)

I want to know more about:

1. unit synergy of different races
2. is there some kind of roles in good teams (like harvester / warrior / man that do map control and harras etc.)?
3. specific build orders

Any short tips/tricks are also welcome!