I believe that Rick's Dakota recurves that he tillers for 3 under are as quiet as most other recurves I've ever heard being shot, whether 3 under or split. Since Rick shoots his own bows, that's probably why he made that comment. Some of the other bows that I have seem noisier when shot 3 under, even bows that I ordered from the bowyer tillered for 3 under.
Black Widow bows generally seem noisier to me shot 3 under, but Rod Jenkins's Black Widow bow, which I believe he shoots 3 under, is one of the quietest recurve bows I've ever heard, quieter than Rick shooting his own Dakota bow or any other recurve I've ever heard. You would have to go to a longbow to get a bow that quiet.
I'm sure that great shooters have great releases, which may be a big part of the reason why Rick and Rod can shoot 3 under quieter than most other people.