Re: what other forms does water exist other than solid, liquid and gas

That depends what you mean by 'exists in'.  If you mean states of matter,
it can be in any of the four states of matter (solid, liquid, ice, or
plasma).  It's worthy of note that ice can have multiple molecular forms,
too (for more info, see http://en.wikipedia.org/wiki/Ice#Types_of_ice). 
Another state water could be found in is its supercritical form (elevated
temp and pressure, to the point that it no longer behaves as either gas or
liquid.)

It can also be found as an isolated water molecule or ion (I'll distinguish
it from the bulk water case because in isolation it loses its stabilization
network.)

Cheers,
K.D.