Congratulations

I had to convince DH about going for number 3. The age gap you are going to have is quite good.
We kept our car..it's a sedan and DS1 is in a booster inbetween the two. We don't have a big car it's a Ford Mondeo and it's a squeeze, but they all fit safely.
You will be amazed at how little one's have a sixth sense about when you are busy with baby. I get them both to help me, include them. They all hop into the bath together
Have you ever heard the saying 'if you want to get something done..ask a busy person'?
It will just click in and you will get a routine happening. Some days can be tough, but they can be tough with 1, 2, 3 or 6.
So you will have a nearly 5 and nearly 3 year old. You can start planning before hand. Perhaps start DS2 with toilet training a couple of months before baby comes along and look at putting him into a bed. Will DS1 be starting school next year? That can take pressure off. I keep DS2 in one day creche so he can play with friends so I have one full day with just DD. Are you in contact with a Mother's Group?
My boys share a room and they love it, especially DS1!
I didn't have alot of help when DD and I came home, DH had just started a new business and couldn't take any real time off, so I was prepared beforehand. Ask for as much help as you need.
You will be amazed how number 3 seems to just go with the flow, they really don't have a choice. Their are times when I have to let her cry while I am attending to the boys, you just seem to cope. The bouncer has come in handy alot more with DD.
You will be fine , trust your instincts you can do it