Oh yeah, and I appreciate seeing some interest in it. You can load any of your HS characters and apply a set of parameters to create a card. With quick card creation you can get a random template for parameters, or you can mod or create any of those yourself. A template is effectively a character class, and defines the base stats, skills, active and passive abilities, and the AI for when the unit isn't player controlled. You can save any character's parameters as a template and apply it to any other. You can also create the character cinematics (cut-ins) and skill animations; those aren't important to gameplay, but they're great for establishing and expressing your characters.
If you want to defeat the preset girls' team, you will need an SX class character. It's a very challenging fight when done 3-on-3, but nothing prevents you from loading a team with the maximum number of SS, S, A, and B characters. A team of 3 that could beat them would have to be strategically developed; I went through a lot of fails before I could win that match.