Opening 17 already made it obvious.
Luffy will fight Akainu to avenge Ace's death, Sanji will fight Kizaru because of how much Kizaru uses his legs to fight, and Zoro will fight Fujitora because both are swordsmen.
The only one that is left out is Green Bull. Perhaps Luffy will defeat Green Bull before fighting Akainu, just like he fought Blueno before fighting Lucci, but I prefer to think that Jinbe will be the one to fight Green Bull.
The only arguments that I think can be used against this is the fact that Kizaru began to use a sword created with light when he got serious in his fight with Rayleigh, so Zoro will have to beat Kizaru in order to be recognized as the best swordsman without leaving the question in the air as to whether or not, even if he's able to beat Mihawk, maybe he wouldn't be able to beat Kizaru. And maybe Fujitora won't appear as an enemy, so with only Akainu, Kizaru, and Greenbull left, it will somehow make sense for Zoro to be the one to fight Kizaru while Sanji fights Greenbull in case Greenbull doesn't use a sword as well.
But I still think it's more likely that Luffy will fight Akainu, Sanji will fight Kizaru, and Zoro will fight Fujitora.
By curiosity, what makes you think that Sanji will fight Greenbull and not Kizaru? And what makes you think that Greenbull is the 2nd strongest admiral? And who are the strongest and the third strongest then?