To my knowledge Sousa never wrote a 3rd trumpet part for The Washington Post march. Yes, the reality is that he never wrote any trumpet parts ... he wrote only for cornets, but even then he didn't write a third part for it. IMO it is the second most popular of Sousa marches ... one that I've been playing since I was a sophomore in high school, under the direction of a former Sousa cornetist. The original composition for it is in the Sousa Museum at the University of Illinois - Urbana. During my comeback, I've recorded it and many other Sousa marches with multi-brass playing all parts. Such is not to say other arrangers haven't twiddled with the original! Even the USMC has.