What exactly were 3-pdrs used for? Is it unusual to find them in an artillery redoubt? (they seem quite small to be of much use for an entrenched position).
To add to Stephen's post, while such light field pieces were disappearing from common field use during the period, they lingered in surprising places, owing, I think, to their versatility in difficult terrain: while smaller and lighter than larger calibres, the long (and realitively heavy) versions could pack a substantial range and punch.
To your question specifically about where used, a British brigade of 6 3-pounders was to be found in a scratch redoubt on the battlefield of Talavara (1809), where they rendered active and useful service.
Okay, that's about the last we hear of them in the Peninsula, but still ...
So service 11 years prior is entirely reasonable.
Cheers - Howie