He can fight Shanks, not denying that, but I personally believe he'll kill Shamrock first, take the fruit from his sword, then fight Shanks and unleash the Cerberus (or some innate ability within him, since it's hinted his body and lineage are special and that he has some ability that allows him to do things like eating more than one fruit), and then gains the upperhand through that and nearly kills him until Luffy arrives and stops the final blow like with Law vs Doflamingo:
Sabo COULD fight Shamrock, or maybe Teach even arrives at the last moment to finish off a weakened Shamrock and take his fruit after their fight, but I'm not certain about that.
His flag has 3 heads on it, he already has two fruits and one more would be the perfect addition to his collection, and he almost definitely would get a Mythical Zoan as his third and final fruit since the other two have been some of the most powerful of their respective categories.