This must be a duplicate (and documented), but I can't find the answer via Google.
In Python, how do I get a datetime object for '3 years ago today'?
UPDATE: FWIW, I don't care hugely about accuracy... i.e. it's Feb 29th today, I don't care whether I'm given Feb 28th or March 1st in my answer. Concision is more important than configurability, in this case.
import datetime datetime.datetime.now() - datetime.timedelta(days=3*365)