Go to control settings and set the same controls for 3 players (yes for 3, not for four, believe me, 3 is best, for all the combos you can do with 3, instead of 4 characters) and connect with someone who did the same. The play survival or stage and move all your characters into a corner of the map, so that they stand all in the exact same place. Then just go on playing and have fun with 3 times as strong moves.
This requires some work when your characters get split up by the opponents, and it might be you lose characters. One more thing to look out for is the possibility to defend against your 3 times as strong moves, if they hit at exactly the same frame. For example a simple Bandit could block 3 times dash, if they happen at the same time.
Some characters simple punches or attacks become infinite combos, because the opponents can't fall to the ground when you attack them and others will cause insta-DOP plus a 3 times a strong finishing blow. You can try a lot of things, which usually with only 1 character wont work.