"Would you go back if you could?" What is this? Please help

Sorry, I don't understand your question.

Elio is asking whether or not Oliver would go back to that summer and start their relationship over again knowing what they know now - and thus not waste so much time. The line "But I've had two of these and I'm about to order a third" reflects back to the time Oliver was at the breakfast table and was asked if he wanted another egg, and he remarks that he knows himself. If he has two, he'll have three, then be stuck at the table all day eating. It was a subtle hint that Oliver still has trouble with maintaining control when confronted with something he wants, but knows he shouldn't have. Much like what he felt for Elio.