if i read this correctly you use the Assist method instead of the Focus method to designate the leader?

in this case make a macro on each char that does

/assist [mainchar]
/focus target

and put this on different key's on each char (ie. F1 on the first char, F2 on the second, etc)

next make a macro on each char for sheeping

/cast [target=focus,harm,nodead][target=party1target,harm,nodead]
/cast sheep

and put it on the sheep button


run up to the mob group
select 1st mob, press F1 (take into account the normal server delay, might press this several times to make sure the character has the right mob focussed)
select 2nd mob, press F2 (take into account the normal server delay, might press this several times to make sure the character has the right mob focussed)
select 3rd mob, press F3 (take into account the normal server delay, might press this several times to make sure the character has the right mob focussed)

now all 3 toons have a different mob focussed
next press your normal /assist [charname] macro to make sure they all target the same target you do, to avoid them breaking sheep on the 1st cast, again take note of the server lag

press the sheep macro, and they will each focus another mob, while they all are targeting the mob you target, so when pressing 2, they dont break sheep

this also makes it easy to Resheep, just press the sheep macro, as long as their focus target is still alive, they will sheep that, if the focus target is dead they will sheep YOUR target instead



NOTE: For people using the focus method of assigning the master target, replace focus with target, and target with focus in the example above