Still waiting for the viz transaltions but my guess for now is:
Oda has always grouped together normal giants -i.e. Dorry & Broggy -, ancient ones -i.e. Oars & Oars Jr. - and YetiCool Brothers as the same big race named Giants
BM knows of the existence of sky people, she's seen Urouge, has intel on a lot of weird races and we've seen there's more than one sky island.
So, if Oda groups all of the sky people subraces as part of one big race, and King is a part of that group, when BM mentions the 3 races she lacks on WCI, she would be referring to
Race 1. A race that is believed to have gone extinted
Race 2. Sky people, of which King is a part of
Race 3. Giants