M.L.
I did have three children under the age of five. I also had four children under the age of five-and-a-half. (Now I have four children in their thirties!) I thought the jump from one to two was harder than from two to three. The only thing I really had to do when #3 arrived was to get myself more organized and give myself more time to do things. Obviously it takes more time by the clock to get three children dressed, to get them in and out of cars, etc., etc. So recognize that your life will be changed in that way, and plan for it.
I wish I had known more about home organization then; it would have helped all of us. I learned later, but it would have helped when my children were tiny.
Think now about how you will get the rest you need. That's crucial to your health and your family's happiness. Is it time to think about getting some help around the house? Even a couple of hours a week put in by a high school kid who's a good cleaner would be helpful. You need to use that time to rest yourself and enjoy your children.
And there are plenty of cars that fit more than four people. We went from sedans to a station wagon - remember those? - to a minivan. We have stuck with a minivan since then so we can carry both dogs and grandchildren (sometimes at the same time). Some people laugh at minivans but they're practical! Our cars may not be "cool," but our kids are the greatest!
Oh, and it's not exactly true about three kids possibly being too much for two parents. After all, God gave each parent TWO hands! So when you're both around, it'll be easy. When it's just you, you hold your three-year-old's hand and your five-year-old holds your three-year-old's other hand, and the baby's in the stroller. It can all be worked out!