Originally Posted by Kuchen

If you just look at the date of the last transaction in your account and add 3 years to that you get the date you are looking for...

I think of it like this (and this may be slightly simplistic, and there could well be exceptions to this logic, but it works for me); rather than considering my Avios themselves as having an expiration date, I work on the basis that my BAEC 'account' would expire if there had been no activity in 3 years, taking all the Avios in the account with it.