I remember reading a similar thread awhile back. I believe the wisdom of that thread was to have a macro as such:

Toon1
/targetenemy
/castsequence Polymorph, ,

Toon2
/targetenemy
/castsequence , Polymorph,

Toon3
/targetenemy
/castsequence , , Polymorph

Bind all 3 to the same key, and press it 3 times.. Toon1 will cast after targetting once, Toon2 after 2, and so on.