When Luffy said he'll beat all of them (and held 3 fingers), he wants to defeat Shanks (his promise as a kid and he wants to fight him even more since he knew he was an Emperor, Post EL). And the reason he he only showed 3 fingers and not 4 is because he doesn't intend to fight BB because of his title but because he was responsible for Ace's death.
So to put it bluntly; he wants to fight Shanks, Kaido & BM because of their title (3 fingers) and BB for Ace.
Seems far-fetchedxd But IMO
So to put it bluntly; he wants to fight Shanks, Kaido & BM because of their title (3 fingers) and BB for Ace.
Seems far-fetchedxd But IMO