Dury said that it was called part 3 because there were plenty of reasons to be cheerful. Therefore calling part 3 implies that theres 2 or maybe more. I suppose he's reasoning was, 'just get the feck on with it'. He also said the only time he ever thought of his polio as being as disability was when he ran for the bus once and missed it !
I had a track on my LP called 'In This Jam (Rio) part II (remix)'. There was no part one and it wasn't a remix, we just did it for a laugh so people would try and find part one or the original mix of part two.