I think it's really a moot point honestly. Crayton tends to be the "slot" receiver and by that alone would be considered the third receiver on most teams.
Really, after the first receiver (Owens), in this offense it really doesn't matter who is the second, third, etc. receiver because all that matters is number of plays .. and the plays called are dictated by the defense more so than the desire to get a specific receiver (other than Owens) the ball.
For example, most of Crayton's plays come in the 10-15 yard range where most of Austin's plays come in the 15+ yard range. If a defense is crowding the line of scrimage and keeping their safeties and linebackers in, then it makes more sense to call more plays for Austin who can stretch the defense. If a defense is playing deep such as a cover-2/tampa-2 style defense then it makes more sense to call more plays for Crayton.
-Reality