Here sort of the idea .... think of the abilities that each class uses while they fight. Forgive if I get spell names wrong ... or if I make them up

Create a macro for tank with:
/cast Grab aggro

Shaman
/cast frostshock

Priest
/cast lesser heal

Now put all the macros on the same action bar button lets say 3. Lets assume tank is your main - create an assist macro for your DPS and put on button 2:
/assist tankname

Assign your healer to target the tank, and put on button 2 as well
/target tankname

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Now for the fun, your tank targets a mob, press number 2 on your keyboard (don't click with mouse, it has to be a keyboard press so it gets to all wow instances).
Then start spamming number 3 - mob dies - something like that.

read more on this link:
Macro's for New Multiboxers - A Quick Start Reference

Key is, you have to use the keyboard, no pun intended. Also i hear that Paladins make better tanks for the multiboxer