I'm getting back into multiboxing myself, thanks to the RAF. I used to play five characters, now I'll have two or three. The reason I quit multiboxing was that it was too many details. My advice is to start simple. Focus on the main. Have for instance these six functions keybound: Accept macro, follow macro, stop (strafe), buff, heal, and nuke.

Heal and nuke could have modifiers, like shift for mana burn, otherwise smite. I like macros that forces the follower to stay on a target until it's dead, then assist the main. Something like this maybe (main is the focus):
/target [harm,nodead][target=focustarget]
/cast Something

Buff should be a castsequence so that you can hit it a couple of times and get Inner Fire for the priest and PW:Fort for both, to minimize the number of buttons needed. There are plenty of macros around here.

Instead of healing the priest, practice getting and keeping aggro with the paladin. You can rez him later if you need to. If really wanted, you could have the priest heal focustarget if friendly, then try to heal focus. That way, when the paladin has a hostile targeted, the pala would be healed. If the pala has a friendly targeted (for instance, the priest), the priest would heal that.

This leaves you with plenty of free keys to play your main character.