I know you asked for opinions on Mom's of 3, but I thought I would offer my thoughts.
Rich (hubby) and I never wanted kids. We met, dated, married and always agreed we didn't want children. So, I thought. After 7 kid-free years together, we changed our minds and I got pregnant within 2 weeks of stopping the pill. Unfortunately, I miscarried - twice. I was blessed to give birth to our son in June 2005. And, after dealing with severe colic for 12-15 weeks I swore I'd never, ever have another one again. Not that I didn't love my son with every cell in my body, but it was so overwhelming for me (us) and such a change from the life we were so used to, that I couldn't fathom another child.
I don't know when it happened, I don't know how it happened, but I wanted another baby. When I told Rich, his response was "Are you sh*tting me?" (he never swears). We talked and talked for about 2 months before we knew it was actually what we both wanted. Our fears were very similar to what you wrote. Do we have enough room? We need a bigger car (had a 2005 VW Jetta). What about the cost of daycare? How will we be able to give Jacob (our first) the attention he's so used to? How will I be able to take two boys to the grocery store, Target, whatever? COLLEGE?!? There were a lot of things that were screaming "don't do it" to us.
Our hearts could scream louder, though. I'm due with our second son at the end of August. :)
My point is that you truly need to look at what both you and your husband want. Listen to each others fears and concerns. I agree with everyone that said you will never regret a third child. My mom had 4 children (I'm the oldest and only one that was 'planned'). She loves us all with every ounce she has. She looks back and says she doesn't regret God blessing her with 4 children, not for a second.
I'm truly done after this one. No doubt in my (or Rich's) mind. We are happy with having two children and are very comfortable in our ability to handle them. But, that's what works for us. When it comes down to it, it's what you and your husband want.