Balck Belts in 3 arts in 15 years may not be so difficult, in fact he may have gotten all 3 from the same instructor. Many TKD schools have a TKD and Hapkido curriculm. How good they are I cannot say. When I was at college (1975-1977), the school there had such a curriculm.
Then, the very same instructor may very well have a Korean / Okinawan roots. thereby allowing some sort of cross certification.
What you point out is noteworthy Stated degrees mean little. A close look at the reume is a better idea. But, in almost 20 years of running a school no one has asked to see mine, with th exception of those ITF people in thearea who may have already found it on the net.