In addition to what others have said, Blackbeard has a three motif. It's part of his flag, guns, beard, and it's clear Oda wants to highlight this aspect of him. Whether this means he has three hearts, souls, is three people, will have three fruits, dreams of three day weekends, it's anyone's guess, but it's a clear thought.
Isn’t that implied to mean he just steals df powers because of his darkness fruit?
Beyond that, you also have to realize that this creates insane narrative problems.
he isn’t special himself
If the "weird" aspect Marco mentions allows him to use two, or maybe even three fruits, it creates a hard limit. He can use three, just like others can use one, which is imbalanced, but not particularly problematic in terms of narrative. However, giving the fruit this limit feels arbitrary. While Oda has added arbitrary negatives to limit overpowered fruits, they usually link to the concept in some way. It isn't just that Boa's power doesn't work on every third Tuesday, you're immune if you don't lust after her. This would be something weird like the void he creates can only hold three powers for some reason and... see what I mean?
It's much better for the narrative to make the limit part of him, not part of the fruit. Especially given this potential power also lacks the foreshadowing to explain it having that ability.
