game of thrones - Who are the three people who could match Jaime Lannister?

Since Jaime himself has yet to reveal who those three are, we simply don't know.

In the books Jaime idolized Ser Barristan "The Bold" Selmy (the Lord Commander of the Kingsguard at the beginning of the series) and has always spoke in admiration of his swordsmanship, so it's safe to assume that he's on the list. Other than that it's pure speculation. Among the living the Clegane brothers (Sandor the Hound and Gregor the Mountain) are often described as powerful warriors even by Jaime, so they might be the other two. There are many others, but they are either dead (Ser Arthur "Sword of the Morning" Dayne), not included in the series (Ser Garlan Tyrell), or haven't appeared yet (Oberyn "The Red Viper" Martell).