"Although the Ultra Beasts are not Legendary, they share some characteristics with numerous Legendary Pokémon: [...], are guaranteed to have at least three IVs of 31 [...]"
All Legendary Pokemon (as well as Ultra Beasts) have been Guaranteed 3 Perfect IVs ever since Generation 6. Before that, Legendary Pokemon were not guaranteed any IVs.
Also, some pointless trivia, but since some Legendaries have 3 Guranteed IVs, they cannot learn Hidden Power as a Fighting type move. :P
Hope I helped!