This can be done in all modes except story mode. Because you cannot have all level 3 characters in challenge mode, use the following trick to win. If you have not unlocked the characters levels for Android 18, Frieza, or Trunks, then do this. If you do not want another character after you pick one character, press Start and choose "Yes" The winning team should be the following:

Have Android 18 level 2 and have Frieza or Trunks at level 2 and the other (depending who you selected second) at level 1.

Have Android 18 level 3 and choose to have just Frieza at level 1 or Trunks at Level 1.

Have Android 18 starts the game and use rapid fire (hold R and repeatedly press B). When your opponent is on ground (you will notice the gauge between players health flashing and your opponent is still on the ground), use the Ex Power Shot (hold R then hold A for three seconds). Repeat Ex Power Shot for three blasts. After third blast, let them have it with more rapid fire. If Android 18 somehow dies, you will have Frieza. Make sure Frieza has 100% Spirit. Some how go above your opponent (probably after he has been hit with a smash downwards) and use Frieza's Special (Deathball). Make sure it does not hit your opponent. If done correctly, the timer will appear. Fast dodge or do whatever is needed to keep Frieza alive until timer reaches 0. Your opponent will die in one hit and you will be still alive. Trunks is just included for speed. If you want to defeat your opponent quickly, use Trunks.