What level are you?
If you're 80-90 Slim, Gizmo and Seraphine.
If you're from 0 to 70, Seraphine, Gizmo and Slim.
Otherwise it's a personale choise...Seraphine is a very good hero till medium level, then you'll find her too squishy and not so usefull, as Giles says. Slim on the contrary, not so good till high level (and at least 6*) where he can shine in the right team.
Gizmo is a different story and depends on the rest of the team, but very usefull to destroy tank as Elric, Torsen, Leon and Drago (vs magic tank it's a bit harder).