You'll be fine! I did it the other way around, the twins were 25 months old when the baby came. It's a lot of work but really lovely to have them all close together in age. Ours are such a cute little gang now and play really well together between fighting over princess dresses.
Survival: don't be shy to accept help when it's offered. Give Home Start a call. They are a charity dedicated to helping families swamped with under-5s.
Car: We have an Smax. It's brilliant. 3 full size seats with 3 isofix points across the middle (fits our three ginormous kiddy seats) and two flip-up seats in the back for when the grandparents visit.
Pram: We had the Donkey, which was a godsend. Bought it secondhand for 700, used it for 4 years and 3 kids, then sold it on for 550, so even though the initial outlay is a lot, they're a bargain in the long run in my opinion. Footprint of a wheelchair, so it fits through doors, lifts, on buses, etc. You can get a buggy board and even a seat attachment to go on it for the toddler. Fits in the Smax boot even with the back two seats up.
I kept mine happy by going to local playgroups. Get out of the house every day if you can; it feels like a huge task, but it takes up time (which can drag with that many small kids at home) and will improve everyone's mood. I found there were almost always old ladies at church groups who saw I was overwhelmed with 3 under 3 and offered to hold one of your babies (to which I learned to always answer in the affirmative
).