Just gave it a run on my mages. It does indeed cast on all 3 at once main casts at the specified coords but both slave toons just cast directly at there feet regardless of minimap ping using the

/script MinimappingLocation(Minimap:GetPingPosition())

Also the coordinates do change depending on minimap zoom for me. If i tell it to cast blizzard at 0,15 say, it will cast on full minimap zoom in but i zoom out it says out of range even tho i didn't move.

Its a good start now if i could get a targeting circle on my main and have the toons auto cast on wherever i choose to click the circle :thumbsup: