I'm looking to book travel to LHR from PHX in March 2016 and recieved the following message "Selected flights depart from and arrive at Heathrow Terminal 3". Now granted it's been awhile since I used BA, but when did things change?
Most likely, they're actually American Airlines flights operating as BA codeshares. With the tighter integration of the airline alliances, who operates which route is shifting around. For instance, some routes from the US to Paris that used to be operated by Air France are now operated by Delta.
Just a guess - I didn't do any investigation.
BA has had some flights in T3 for a while. However, this article may explain your PHX flight:
http://www.headforpoints.com/2014/11/17/british-airways-timetable-terminal-1-terminal-3-moves/
All BA flights to/from the USA presently use T5, but they do also use T3 and until the end of this month T1 (which has a handful of residual flights until its closure and is earily quiet to anyone who rembers its heyday).
The Phoenix flight though moves to T3 in October as does Las Vegas and Denver.
Thanks for the replies