I was in your shoes about 18 weeks ago. I always thought I'd have 3 kids, but not for at least a years time. Then our little surprise bean came along. And tbh, it took about 12 weeks to get my head around the idea that I CAN cope with 3 kids, and that I will do it. Yes, there are many things to factor in with regards to going from 2 to 3. We have an 08 Rav4, which at first we didn't think would be able to fit 3 child seats in. So we've done our homework and we've been able to find 3 seats that will fit suitably. Even though we have a 4brm home, one is the spare room/computer room, so my 2 eldest girls will have to share a room. We're in the process now of getting that set up for them.
When this bub is born, my girls will be 4 and 2 & 3 month years of age. They are very active girls, and keep me on my toes all day long.
Do you have much support where you are? Friends? Family? If so, don't be afraid to ask for help. My mum will be taking 2 weeks off in August to help out (mainly with the girls more than with me) and hubby will take 4 weeks off at the end of the year.
Good luck. You can do this.