A third Renown as fast-carrier protection

Would a third Renown-class battlecruiser have mattered?

Yes. A modernized ship such as HMS Resistance would have been a powerful fast-carrier escort and surface deterrent.

HMS Resistance was the intended name for the cancelled third Renown. If modernized like Renown, the ship would combine carrier-compatible speed, heavy 15-inch guns, and ample space for anti-aircraft weapons. An enemy battleship or battlecruiser might defeat it eventually, but would have to fight under fire while the escorted carrier launched aircraft. Treaty retention could also leave the Royal Navy with Hood and three Renowns as a complete fast squadron, probably at Tiger's expense while Nelson and Rodney remained. Destroyers could make smoke and protect the carrier instead of attempting desperate torpedo attacks. The combination of a fast capital ship and a carrier would form one of the period's most powerful naval groups.