about 3 days ago i got two 3 week old girls, alby and birch. alby is getting on really well and even eats out of my hand occasionally but birch is extremely shy and i'm worried i might have really scared her by trying to pick her up

any advice or consolation would be very much appreciated as i just want them both to be happy and healthy!
Hi there, how lovely to have some pups! The fact you have put your worry on this forum shows how much you care.
They are very, very young. 3 weeks is soon to be removed from mum so maybe the world is totally overwhelming? (supply lots of hide outs and possibly even a heat pad for comfort).
Pigs natural habitat is in a burrow. I often put lots of cardboard boxes in my pig pen for them to re-arrange into a burrow!
It will be likely, that outgoing Alby, will help to build confidence in Birch by demonstrating her interactions with you over time.
The most special thing to me regarding guineapigs, is the emergence of individual character. Like people, I have observed over the years, how my pigs have been so beautifully unique. Miss Ginge the Peruvian was shy, whist Margo the alpaca, sensitive to touch, Ted the Rex, imprinted onto me and followed me round the house, George the Abyssinian was feisty, Peggy the himilayn expressed her pig power, Lenny likes a head scratch and Mr Grumbles ... (both teddies) well Grumbles, unless I’m upset and he has a sensitive knowing and calmness allowing for me to pick him up and hold him still under my chin without a squeak - until his presence has healed some of whatever is going on for me.
Trust & Time is super important. Guineapigs in my view are the truest of vegans! As prey animals they are super instinctive. It can take years sometimes to build mega trust with a skittish pig. The longest I have had the same pig was about 9 years.
My current boars took approx 14 months to mature and adolescent behaviours were evident. Like people, you likely get more outgoing pigs and sometimes some less outgoing critters. always let your pig approach you and freely give it the respect it deserves.
Watching your pigs from a distance and see how they react together. This can be immensely rewarding and informative. For example, it’s good to know who’s at the top, who hogs the food and if one is being bullied a bit too much. Etc.
I’m sure your doing a great job. Pigs can forgive. If they didn’t, claw clipping would be impossible but that’s for another time ...
Enjoy your new musings.