Jamie claims that there are three men in the seven kingdoms that could beat him.but there are several he probably didn't consider,Qhurin halfhand was probably the best fighter in the nights watch,Jon had a lot of trouble fighting him and Qhorin was holding back and Oberyn almost beat the mountain but him wanting the mountain to confess caused him to drag the fight out.I doubt Jamue had even heard of Qhorin and I don't know if Oberyn was one of the three but for some reason I doubt it. EDIT:after reading the comments I've come to realize that Qhorin would probably not stand a chance so I'm replacing Qhorin with Greatjon Umber,Yohn Royce,Garlen Tyrell,and Syrio forel