Are you by any chance Tuco from FoH?

Anyway questing while multi-boxing can be a chore in legacy content. It gets more and more streamlined for multiboxers as you move through expansions. Once you hit WoD questing becomes much better. That's where all quest items can be gathered/looted by multiple characters at once. There are some annoyances like phasing that become an issue in Cata and on but nothing unbearable.

3 Boxing is doable without it becoming frustrating. I'm not sure I'd try to 5 box a team from 1-110 via questing.

When you see teams that are all one class that is just for ease of setup and play. However, many of the people here and in the ISBoxer discord use mixed team Trinity setups with a tank, healer and 3 dps. WoW is one of the easiest games out to do this with because of their addon and macro system. As for what classes to choose? Well if your main concern is seeing the content you missed while being away then really any class comp is going to work for you.

As for ease of macroing rotations some specializations are much easier. BM Hunters, Frost Death Knights, Havoc Demon Hunters, Ret Paladins are probably the simplest. Affliction Warlocks, Fury and Arms Warriors, Assassination Rogues and Windwalker Monks are not much more difficult to set up reasonably well. Balance and Feral druids require a little more work to set up good rotations for but are doable. Enhancement and Elemental Shaman I find to be a major PITA and all other specs I don't really have experience with outside of using them in same spec teams where I am playing the classes normally and not using macroed rotations.

Windwalker Monks are incredibly powerful even at low gear levels once you get to end game. They have very strong cooldowns on very low timers. Affliction Warlocks are also incredibly powerful by virtue of their top tier sustained damage + self healing. And they get pets that can tank for them as icing on the cake.