3 Dates And He Expects Me To Be Exclusive To Him? Leo and Rebecca Buzz Question

Leo and Rebecca Buzz Question...Been on 3 dates with this guy and the other day he saw me out on a date with ANOTHER guy. He calls me that night and says WHAT THE HECK..I thought we were dating. He said after 3 dates..it made us an official couple...and I really shouldn't have gone out with someone else while dating him...Wow..really??? Since when does 3 dates make us 'official'???