We put 3 lines in a 3 man tourney with the WCO layout. The trick to 3 man is stay aggressive, unless you are down bodies.
What we did was got to our bunkers, shot a bit to get them in their bunkers, then move. Keep communicating and let your team know what you see. We found it was best to have 2 guns up off the break laning...laneing (apparently that's not a word) and try not to make corners. Make your primaries, then work your way out to the corners to prevent runs from the highway. Again, once you get them in their bunkers...MOVE. Theres no reason to not push up fast and try to get them off guard. Our lines took 1st, 2nd, and 5th (out of 17) using this technique.
If you find yourselves down a body/bodies, stay calm and try to make it out to a corner. If possible, cross your shots so you are each looking for run throughs on each other. If you are by yourself, shoot tape, then look inside, then shoot tape. Don't try to be a hero. If you are by yourself, and you are using a time limit format, just try to run time out.
Depending on the number of teams be prepared to make subtle changes to your breakout, lots of teams will watch to see whats working and what isn't. If you are shooting people off the break (very easy to do on the WCO layout) be ready to send 2 mid and 1 d side, or vise versa to keep teams on their toes. You will start losing every game if other teams find out what you are doing.
Most importantly HAVE FUN! It is a tournament so you will see some pretty good competition. Don't get discouraged if you go on a losing streak or if you place toward the bottom. Consider it a learning experience. You and your team will take a lot of knowledge out of your first 3 man. It is completely different from 5 man. Keep pushing hard and have fun