Congratulations!! This sounded very similar to me!! Right down to how the heck did I even conceive - I came to the conclusion I was getting a bit older and chucking out eggs at random times of the month 😂.
it was a shock but when that wore off I just got on with it… I actually found it easier than I though, especially once youngest went to school too, I had loads more quality time with number 3 and really treasured those special cuddles and chilled a lot more really! I roped the eldest in (6/12) to help fetch things etc and found the other 2 were old enough to walk everywhere while youngest was in pram / push chair which was fine! He had ready made entertainment too, which he loves - he’s almost 13 now!
Another thing I found was that I knew most of the parenting new baby / toddler / weaning etc so didn’t stress as much.
in fact, I didn’t sweat the small stuff as much in general and I had a lot more perspective on things! I already had some mum friends etc and although most of them didn’t have 3, I didn’t feel the same pressure to HAVE to go to every baby group and coffee morning. I did got yo some baby things but I did feel like all mist of them talked about was first time labour and caring for a baby stuff which I obviously already knew.
I used to enjoy lots of walks, feeding ducks, chattering, little trip around the local shops etc, park etc. he didn’t miss out on the baby groups as went to nursery 2x week once I went back to work.
honesty, although it was a shock (found out on New Year’s Eve too 🙈!!), it was not as hard as I had initially imagined.
also, bonus ready made playmates and still are a tight little bunch.
wishing you loads of luck with it all xx