In addition to the special order Mike mentioned, the 3" barrel was standard in the Model 36 with both round and square butt (frame was different, not just the grips). Later they went to a round butt only and used wood filler type grips for the square butt.
There was also one made (special order?) that had a heavy barrel and 1/8 inch sights (vs the standard 1/10"). IIRC, they came only with a steel square butt. I think that one made sense, but I could never figure out who wanted target sights on a Model 36, but I guess someone did.
I never heard any of these called a "combat special", though.
Jim