To truly be efficient, focusing on 1 attribute is essential. I played 3 mages - some versatility but mostly no defense but a huge offense. I could overpower any enemy and use my offense as my defense.

3 Warlocks work the same way, longer to kill but more mobile since dots are instant and do not need to face the target.

1 Paly/2 Mages feels like more work than payoff. It's also a lot easier to manage multiple of the same class, almost like playing 1 character just pushing 3 buttons

3 Priests I got to lvl 61, I didn't enjoy it as much as the Mages.