Guybrush, try not to panic. I have 2 year old twins so I can answer some of your questions, but they were my first children so can't give much advice on juggling with an older child.
Firstly, does anyone know if we should be doing anything any different from a single pregnancy? Taking extra folic acid or anything?
I honestly can't remember. I think I took quite a strong dose of iron from quite early on in the pregnancy, and that seemed to work well as I was never anaemic. Make sure you're eating well (or trying to, if you have morning sickness). The demands are your body are far greater than a single pregnancy. I specifically remember reading advice to drink plenty of water and lots and lots of protein.
My other biggest concerns, less than 24 hours after finding out are:-
The slightly greater risks of something going wrong. I was having a reassurance scan at 7 weeks due to a missed miscarriage a few months ago- but I don't feel enormously reassured!
I can't tell you there aren't any risks, because there are, but there are also many, many women who have easy twin pregnancies and deliver healthy full term babies (37 weeks for twins). Try not to worry and see it as a positive that you will get extra monitoring throughout the pregnancy that will pick up any problems early.
Practical worries- I work 3 days a week and I love my job but having twins plus dd in nursery will be costing more than my salary!
By the sounds of it, your toddler will be eligible for the 15 free hours once you'd be back at work, which would help a bit. I don't know what your household income is, but you may find that you'd be eligible for some tax credits if your nursery fees jump up. If it's really not economically feasible for you to return to work straight after mat leave, does your job have the option of a career break? That way you could return a year or two later when the twins would also be getting free hours and your oldest would be in school.
How in earth does one feed two? Will they sleep at the same time? How will I do bedtime for a 3 year old and two newborns when I am doing it alone (which I am approximately 50% of the time)? Will it be ok for the twins to share a smallish bedroom for their entire childhood? Do we need a new dining table as its only fits 4 around it? How will we ever afford to go on holiday again? I don't have a car, I cycle everywhere. But I can't fit 3 kids in my bike trailer! So I think I am going to have to learn to drive and beg, borrow or steal a car! With 3 fucking isofix attachments! And overwhelmingly- oh my goodness, how the hell are we going to cope? DH and I are I think, strong and very loving parents, we we found the newborn phase very tough with just one.
Don't worry about all of this at once. You can feed two at the same time, whether that's breast feeding or bottle. The best chance you have of them sleeping at the same time is to make sure you feed them at the same time. I do bedtime alone every night with my 2 as I'm a single parent, and you cope. Sometimes it's a mess, but they go to sleep eventually and you try again the next day.
On a practical level, try and get as much help as you can. As well as family members, it might be worth checking out if Home Start offer any help in your area - they often help with families with multiples, and with you having an older child as well I'm sure you'd qualify.