For Cornelia I'd presume you should just have her as either Gremory or Warlock, since she's more of an Attack Caster.
For Ladislava your best bet would be ranking her up into Wyvern Lord, since it's practically the same with the added benefit of Avoid +10 and some stat / growth changes.
For Randolph, either keep him as a Warrior or rank him up into War Master while still using an Axe.
If you want to have them at a lower level and "progress" using them, I'd recommend (from personal use);
Cornelia - Monk, Mage, Warlock, Gremory
Ladislava - Soldier / Fighter, Cavalier / Armored Knight, Wyvern Rider, Wyvern Lord
Randolph - Fighter, Brawler, Warrior, War Master
Obviously class choices are up to individual preference, but that's the route I'd take with it.