Okay this is starting to irritate me so I'm making a tip just to clarify and clear things up. YES, THE THREE TITANS ARE BOSSES. YES, THEY ARE TAMEABLE. STOP ASKING. Also, no, YOU CAN NOT TAME THE KING TITAN.
Pro tip: Don't use explosive weaponry and really bad rexes with no saddles like the devs did. Fab Snipers for taming it with little/no collateral damage, actually decent rexes + imprint + saddles to kill it