And please don't feel you should be going anywhere or doing anything just because others do. Not every new mum will be going out all the time, and of the group of mums you mention, someone else may be feeling just like you but doesn't want to admit it.
Dc3 is 10mo now. I'm only just starting thinking I should probably start taking him to a couple of baby groups of some description, swimming and the library probably to start with. He's a happy little baby, but when strangers shout in the pram at him he does get upset (don't blame him, I would too!) so I think interaction with other babies where it will likely be loud, may well now be of benefit to him. Don't think it would have before, and to be honest, I just love being at home with him!
I tried a couple of toddler groups with ds1 and dd and found them so cliquey I never bothered again.
So deep breath, and just take one day at a time. Maybe on a Sunday think of one or two things you might do that week and plan how you can make them easy and enjoyable.
Oh, and if you don't already do it, now is the time to start online shopping for your groceries
(well, and everything else if it makes your life easier!) that way, the supermarket can become a place to pop to as your trial runs. They've always got accessible changing facilities, cafe, and usually a few crying babies, so nobody bats an eyelid. You could maybe pop in, buy a magazine, bubble bath, chocolate (basically a treat for yourself!) and know you can leave at any point if you're not comfortable.