A rather surprising order TBH when BA are about to become a relatively large user of the A350-1000 - which is pretty much a direct replacement (and considerably more modern) for the 777W.
Like for like, the 777W is now a generation behind the A350-1000 so with the industry replacement for for the 777W on order from Airbus and deliveries starting soon, why buy more of the old generation? Unless a better delivery / dumped price for end-of-line frames. Probably both!
Looks like BA is not buying these 3 B777-300er's, but leasing them. I think this will get them early delivery, but the length of the lease has not been announced. It might only be 5 years, or it could be much longer. Time will tell when more details get released.