How to remove one step from a three step ladder?
Note the slots cut in the holes at which the supporting rods attach. The rods have a matching tang. It seems likely that you can fold the ladder in such a manner to allow the tang on the rod to align with the slot on the step which should then allow the rod to slip out.. 

It may be necessary to remove the other step to accomplish this, or you may be able to remove the desired step straight away.

Detail is missing for the main support rod.