Would you mind sliding your seats forwards and looking at the very base on the back of the seat for me? perhaps with a few pictures?
Almost every 5 door car I've had the privileged (yer right) of working on, that also came in a 3 door variant used the same seat frames, they just left the lever and cable off the 5 door. This is never noticed by anybody, but it normally means that 5 doors have a hidden feature, you just need to know how to activate it, which is normally a matter of flicking the hooks at the bottom of the seat over manually.