The reason that won't work is that Thunder Clap will cast whenever the condition on line 1 returns false. What you want is:
Code:
#showtooltip
/cast [nomod:shift,form:3]pummel;[nomod:shift]thunder clap;[form:1]rend;piercing howl
i.e...
"cast pummel in form (stance) 3 without shift, thunder clap in form 1 or 2 without shift, rend in form 1 with shift, piercing howl in form 2 or 3 with shift"