Hi C.,
So, I did a little research after reading your question and the responses. You didn't mention how much your 5 1/2 yr old weighs or when she will be 6, but I found a link about the car seat laws in California that might be helpful. Basically, if you can't SAFELY fit all three seats in the back seat, you CAN put the oldest child in the front seat as long as you can turn off the passenger airbag. (if it has one.) Kirstin L is wrong that it is against the law to put any child in the front seat. There are certain circumstances where it is allowed. We had a similar issue when our family car was in the shop and we were left with the pickup as our only car for a few days. We called our good friend who is a CHP and he gave us the same info that is in the link that I'm going to put at the end of this response. I too suggest taking all three seats/boosters to the CHP or the local fire station and have them try to help you install them in the back properly. Buying a new car isn't always an option for some people. Good luck and let us know how it goes! Congrats on your soon to be born baby!
Sincerely,
L.
http://www.inventiveparent.com/lawsreg9.htm#California