I never used Relm. Mostly because her special ability is mostly a piece of crap. (and apparently really buggy as well) If you've got two magic users, that's probably plenty. Throw in someone else that can pack a punch, because locke isn't too good at that (shadow or sabin would be a good choice probably)

Make sure you keep switching out espers though. That is one of the most important things to do (I'm not sure if you've got any yet, but when the time comes) They're your way to levels, and when you've learned all the spells and don't need a bonus, give it to someone else. And when you have all the spells/abilities/whatnot learned, don't try too much to round out your characters. You can do that easily enough with equipment and picking the right people (you have like a million people in your party anyways) So yeah, be wise in your esper choosing.