What if he had already eaten another devil fruit ?? It has been shown in both Anime and Manga that under Marshall D Teach possession there is a special kind of book with a whole lot information about Devil Fruit in it ..
Maybe there is a special kind of devil fruit a Mythical Zoan or a special kind of paramecia Devil fruit maybe ( A possible name for the could be: Inu Inu no Mi, Model: Cerberus. ) with ability like Cerberus the three-headed mythical creature.. which will let you have three bodies merged in a single one. We have seen many kind of special devil fruits, as an example you can take Brook.
The Yomi Yomi no Mi is a Paramecia-type Devil Fruit which enhances the user’s soul to the point where they can come back to life after dying once, remaining alive until their body is sufficiently damaged. Then why not a special devil fruit which allows the user with three bodies . As no three is actually fits with him in every way possible.
The Jolly Roger, or the Pirate Flag, help determine who belong to whose crew. The designs usually reflect the interest of the captain. At other times it may be physical resemblance or the symbols behind them. In some sense, there is a reason or story for every Jolly Roger design.
For some reason, the design of the flag of the Blackbeard Pirates show three (3) skulls side by side. There hasn’t been any explicit reasons for this but it might be revealed. Other than that he always keep three guns with him, has three names.
Now take a look at this video.. and please watch till the end … (This speech is epic though, I love it <3 )
In episode 146 Luffy says: it was not “that guy”
Zoro adds: it was “those guys”
that mean they probably knew that there is more than one person resides in his body…
And even before Oda introduces Cerberus to us.. here some manga portion in case if you missed it
In Chapter 444 there was shown a Cerberus in Thriller Bark.
There was another instance wherein a Cerberus was mentioned. This was during the CP9 Arc in Enies Lobby courthouse. In Chapter 387, during the Battle between the Franky Family and Baskerville, there was a comment on how Baskerville resembles a Cerberus.