Take Oven, for instance, his beard is similar to that of Kaido. His face structure is also rather similar to his. What about Katakuri? His hair and eyes also do resemble Kaido quite a bit. And also his enormous strength as well. As a son of Kaido, this would explain a lot about his enormous strength as the most powerful crew member of Big Mom Pirates.
Everything lines up perfectly too, the three were born over 40 years ago, the same time Kaido and Big Mom were an alliance. And we see that when they were young Big Mom travelling around as indicated by their surroundings. That allows the roam for this speculate this. Another evidence that confirms this theory is that I always found the name of these triplets very odd. Potato starch is “Katakuriko”, yet Oda simply used Katakuri, without the ko. Now, this isn’t really a big detail, bit you’ll see what I’m getting at in a bit.