At level 74, they can still get exp off of any mob lvl 65 and higher according to http://www.wowwiki.com/Experience_point So if you want you could try running a lvl 65 instance. Depending on the roles of the 74s it could be a cake walk.

But I'd just do mad quests on just the 3 toons to close the gap a bit. Get them to 65, and hit up Nagrand, do the Ring of Blood questline for some nice weapons, and you should be doing fine. I'd keep the 74s in the same zone as your lowbies, and bring em in for group quests and instances, but leave them out for the bulk of the questing. Once you close up the gap, and if there's any outland zones your 74s havent done yet, you should group them all at once. Why lose out on the xp and gold from quests if you don't have to?