Congratulations to you both!
My 3rd was also unexpected - but he's the best thing that could have happened, he is the most placid, happy baby I've ever met. The complete opposite to his 2 highly strung sisters! I had the girls 21 months apart, and thought my nappy days were over, but then he came along when my eldest was almost 5. The transition from 2 to 3 was nothing like i imagined, it was much easier. I think with 2 older sisters he's used to waiting for things, and he's had to fit in with out school runs and other routines.
I would say don't panic - everything happens for a reason, and if your 3rd's are anything like mine you'll be fine.