It's given by Oda Full stop.....that's it
How did they got is beyond "us mere reader" could possibly answer because Oda himself can't answer either or doesn't focus on such things.
There is no evidence that Mihawk beat any top tier who uses swords( Shanks, Fuji, Ray, etc)
Same for WB and Kaido
They doesn't seem to earn those titles by participating in a competition and beat all possible fighter.
News of top tier beating another top tier in one piece is almost nonexistent ( only Akainu vs Aokiji)
These titles are given by the people of the one piece world in other words Oda.
In what way?? Nothing but a person's individual capabilities and reputation(mostly capabilities)
Mihawk got blackblade + supreme weapon, according to him it's also strongest sword. He's also very skilled master swordsman. You don't need too much thought why he is given the WSS
Whitebeard got incredibly physique and was abnormally huge for a normal human, hence got enormous vitality + monstrous strength + fruit that can destroy the world + strong haki, he's the peak human being in op. Hence, not surprising why he's called WSM "NOT" Roger
Kaido is an oni, a creature in Jap folklore that is a hulking figure a monster having monstrous strength, a being biologically far superior to human. He's the absolute unit among one piece character that has all powerful solid trait, somebody that is build different
Hence, having been called the strongest living thing, 1v1 bet on him etc.
