Its easy to over think this stuff. As pointed out above the operator will nominate the approach category based on approach speed in landing configuration. The operator will likely consider the lower approach speed when two landing configurations are available.
This wont limit the crew to conf full ( for the Airbus example ). As crew we should be familiar with the speed ranges for each segment of the approach and comply with them. This will ensure obstacle clearance based on the procedure design. So using Conf 3 for A321 would be possible as CAT C so long as the Initial / Final / Circling speeds are flown correctly. (For PANS OPS Procedures anyway as TERPS procedures dont have the same speed requirements ).