Mapping different actions up is one of the things I find hardest about MB (and I only play two toons at once), bottom line is any ability you want to use on your alts will need to be activated by a keypress (as transfer of mouse position tends to not be accurate).

So, you'll probably end up using less abilities than you would for a solo character of each class. How many depends on your keyboard skills and how many buttons you can reach at once (multiplied by modifier keys). I can reach 1-5 comfortably, which rather limits the amount of actions I can perform easily on my toons, I really need to learn a better set up for my keys...

In your situation I'd have either the shaman or priest as the healer (as their the only options :P) and the others as caster dps.

Macros could look something like this (very simplified version):
Anything starting with / is a macro

1. To get targets on alts (you can also use focus to target, I'm sure there's info in the wiki)
Main - blank
Healer - /assist main
DPS - /assist main

2. Attack sequence to conserver healers mana
Main - Aggro sequence (sunder armour, heroic strike, etc)
Healer - blank
DPS - Nuke sequence (fireball, lightening bolt, mind blast/mind flay)

3. Attack sequence with simple healing
Main - /click ActionButton2 (or something similar, this will activate the macro on button 2 when pressed, but I'm typing this from memory)
Healer - /cast [help][target=targettarget,help] Healing Spell/Sequence (this will cast the desired heal on your target if it's friendly or if you have a mob targeted will cast on that's target)
DPS - /click ActionButton2

4. Attack sequence with healer dps
Main - /click ActionButton2
Healer - Nuke Sequence (lightening bolt, mind blast/mindflay or smite)
DPS - /click ActionButton2

5. Crowd Control (you'll need a macro elsewhere to set up your focus and a way for each of yuor toons to seperately target or focus mobs)
Main - blank
Priest - /cast [focus,harm] Shackle Undead
Mages - /cast [focus,harm] Polymorph
You might want to add some additional options here if you only need one toon crowd controlling.

These would probably need to be much more complicated for instances or higher level stuff, but hopefully give an idea for how you could map up different actions between characters.