Well, the typical approach is to pick a trio that (a) fulfills as many standard roles as possible and (b) can all be played by you.
Back in the day, I used to trio (not three box) LDoNs using my bard and a friend who duo'd a ranger and a druid. Ranger tanked & DPS, druid healed, buffed and nuked, and the bard... well.. I did pulling, slows, haste, mana regen, crowd control, melee, etc etc
This worked until we hit lvl 63-65 dungeons where the mobs were just too much for the ranger to tank effectively (at least at her gear level).
Obviously I doubt you want to box a bard, but you get my point. I'd stay away from "one flavor" characters like warriors, rogues, wizards and clerics and try to maximize utility and roles. Buffs you can get in PoK or even via potion now. Paladin would seem a good choice until the high 50's when lulls cap out (unless that's changed), shaman is a no-brainer and perhaps a druid for heals, nukes, snares, etc. Little light on DPS though.