SOULCALIBUR

Play with three characters in any mode is pretty self explanatory.

You can go onto the Team mode and pick four characters, getting this instantaneously.

As the wording is a bit confusing, I'd just like to make it clear that this achievement doesn't require you to use 3 different characters in any one game mode - you can use one in arcade, one in survival, and one in time attack and the achievement will still unlock. So the best thing is to simply let this achievement happen naturally as you go for the others.