I recently emailed Mercian with a similar question. Their answer was: the last two digits represent the year ('87 in your case), the prior digits would indicate yours was the 277th frame completed in that year.
No wisdom to add as to your "3"
Christian Fritze (Alameda, CA) ---
Mark Poore <firstname.lastname@example.org> wrote:
> Would anyone here know how I might be able to find out the year my Mercian was made? It's serial number is: 27787 3
In case the number doesn't appear the way it does on the bike the 3 is under the 27787. I believe it was made in the early 80's, but I would like to fix a date to it.