3rd cascaded Dropdown - help

Hi, i have 2 Dropdowns, and they depend from each other(has_many).
i used collection_select, observe_field to create the second dropdown,
which gets populated correctly. Now i would like to have the 3rd one,
which is of course as well depending from the second one.
the solution i use is calling a action from the controller, where my
variable gets created @..Find.. and i call then a partial. i would like
to extend the partial to contain the same code as the initial page eg.:
the collection_select part and the observer as well the div-area where
it gets finally replaced.
any ideas why this is not happening?

thx in advance

tom