My older 3 weren't the same age gap as yours (they were a bit closer in age), but we found that 3 car seats fit easily in a Citroen Picaso. It was a relatively cheap car but was very roomy, with a large boot. It saved us having to splash out on a people carrier.
I found I had to think carefully about outings to ensure none of them had the chance to run off, but that probably won't be such a problem with your older two.
My third slotted right in to the routine of school runs, toddler groups etc. she is always a bit sad when she is by herself, she loves having her older siblings to occupy her (she is 2 now). I found things just kind of worked themselves out as we went along.