This would force at least three mobs to be targetted:

/targetenemy, /targetfriend
These commands cycle through the specified type of unit. /targetenemy is like pressing TAB, and /targetfriend is like pressing CTRL-TAB. You can also add a parameter of 1 which reverses the direction of the cycle (/targetenemy 1 is like pressing SHIFT-TAB).
Note: You can only use these commands once per macro.

/targetenemy
/targetenemy 1

So on the main guy you don't use this. On shaman #2 you use /targetenemy. On shaman #3 you use /targetenemy 1. It might be enough to alleviate the problem you are having.