Someone who fails the first year of college/Sixth form and as a result have to do another year just to complete their 2 year course.
A 3 year bastard is the pinnacle of time wasting and no matter how far you go in life you basically died one year earlier than you should have.
In some circumstances choosing to do another year to complete a course you didn't know you wanted to do until the end of your first year can be included in the definition of 3 year bastardry.
Adam: Hey, you going uni next year?
Chaperzz: Nah, man I need to complete chemistry A level.
Adam: What!? You 3 year bastard.