The best site for carom billiard history is undoubtedly Charles Ursitti's. He has all of the tournament results gathered together. See: http://charlesursitti.com/ and then click on History of 3-Cushion Billiards 1930-2010.
But to answer your question more directly....
Raymond Ceulemans is a player from Belgium who dominated 3-cushion starting around 1960 and continuing until about 1985, at which time some of the competition had caught up to him. He won his last of 21 world 3-cushion championships in 2001. For more about him, see this Wikipedia article: http://en.wikipedia.org/wiki/Raymond_Ceulemans But Raymond has never lived in the US.
Sang Chun Lee won 12 consecutive US championships starting in 1990. A couple of those wins were close, but mostly he was not really challenged during his peak. See also http://en.wikipedia.org/wiki/Sang_Chun_Lee He had been the Korean champion before coming to the US.
Each year the program for the US Nationals lists the previous winners. Here is the most recent list of winners including their averages: