i'd say you're looking at a transplant down the line. a norwood 3 is pretty severe in my opinion.
in my opinion, propecia can thicken up what you've got, and also reverse the recession somewhat, but this is why it's important to get on it the INSTANT you see yourself thinning/receding. This way you can restore your full hairline.
If you wait until a norwood 3, the best you could probably do is get it down to a norwood 2 or something... and even that is a long, long shot. in other words, you wouldn't be able to reverse it so much that other people wouldn't notice you're losing hair- even at a norwood 2, you look like you're losing hair.
nonetheless, it's best to get on it now.
and yeah, a hair transplant is what you're looking at, unless something else comes along soon, which seems to be the case