I don’t think Kaido or Marco has the 3rd devil fruit power Blackbeard wants in the form of a Mythical Zoan type. Marco already fought him and lost and as far as we know Kaido can’t be killed.
There is a lot of symbolism in One Piece and it plays a huge part in the story. One thing most agree on is that Blackbeard will have 3 devil fruit powers mainly based on the 3 skulls his Jolly Roger has.
Most also conclude that the last devil fruit will be a Mythical Zoan type and I agree. I also believe that this power has also been shown in the manga already – The Hebi Hebi no Mi, Model: Yamata no Orochi.
Assuming that Blackbeard will, in fact, be able to consume 3 devil fruits and based on the bones on the Jolly Roger – which make 8 points, I think his Jolly Roger symbolizes that he has 3 devil fruits and has the ability to have 8 heads.
Could be a reach, but I think Kurozumi Orochi will play a bigger part in the story than we think, especially as more of the powers the fruit grants him are shown.
*Theory by greecemonke