I'm not sure what you mean by par 3. There is no par 3 course at Disney. If you are referring to the Oak Trail walking course, that is a 9 hole par 36 course.
Yes they do have club and pull cart rentals, but I do not believe there are any discounts for them.
DVC golf membership cost $50 and the rate to play Oak Trail would be $20 for adults and $15 for juniors.( 17 and under when accompanied by the member)
The normal rate is $38 and $17 which was good till 10/4, they don't have any rates posted for times past that, so I'm not sure if the rates go up in the winter like they do for the other courses. Replay rate is 50% off the 9 hole rate.
So it would only make sense to purchase the DVC membership if you were going to play more than once with several players. Do the math.