well, you have to realise when you have only 3 arms, things get really complicated and instead of 2 possible solutions youll get 300 solutions, because the arms will have lots of range of motion.
if you want more than 2 arms on an ik, just make a seperate one for each task, youll have way more control like that anyways. i can probably make a faux 3 arm, but it wont choose the optimal solution, itll bend more like a finger does.
heres the cap for my 2 arm, it uses a trignometry to work
http://willhostforfood.com/files3/6035270/RealIK.cap