If the paladin will be a permanent part of the group let him plvl the 2 alts; if you're just using him to boost, you'll have a third slot to fill and he can't powerlvl something on the same account.

If you want a Trio that doesn't include the pala, start 'em all together and lvl them together; sure it won't be as smooth as the 70 plvling/tanking an entire cathedral of mobs, but you'll learn better how the lowbies work strategy and skillwise, as well as how they work together, you can get a good gauge for dmg output and learn to heal yourselves early on rather than late(like I did with my druids...hoo boy is healing between 3 a pain to learn lol)