A photograph taken by a Boston Marathon spectator shows the Suspect #1 as he walked past the Forum restaurant on Boylston Street (where the second explosion occurred). That image shows that the back of the man’s cap contained four white figures, likely "B330." The black Bridgestone cap does those numbers, a reference to the company’s B330 golf ball.
Why does Ralph Lauren put a #3 on their cap? "The embroidered number on Ralph Lauren Big Pony pieces traces its heritage to the traditional uniform of professional polo players. The players are assigned a position, which is designated by the numbers from one to four and displayed on team jerseys during a match. The player in position number three is usually the most experienced player on the team."
Some speculation was that the individual in the yellow rectangle was Suspect #1, but that appears to be incorrect.