There are many variables in a basement rehab and what the value of it may be. in General I would say a basement rehab is worth about 1/4 to 1/2 of upper level finished space.
A "third bedroom" would definitely increase value but it needs o be a true bedroom. It would require a closet and a second egress.... There are window size requirements that would give it a second egress.
Obviously the fit and finish of the remodel as well as if the back may be a walk out with full sized windows will also effect he value.