So gates are usually numbered by a terminal (A, B, C, etc) and a gate (1 - xx). So his gate was C3, the door through which he got on the plane. Seats in a plane are numbered by row (1 - xx), then a letter designating which seat across (A, B… usually to F or I, depending on the size of the plane). Both designators are on your ticket. He mistook his gate, C3 as being his seat, 3C.