Britax Frontier 85 vrs Radian XT?

Because of financial issues I won't be able to purchase a bigger car. I am expecting my third child in october.. I have a 4 yr old son and a 2 yr old daughter.. I also have a small dodge caliber, so my question is which car seat is the smallest width wise? I would need to buy 3 and fit them in my car.. 2 front facing and one rear facing of course for the baby I wouldn't buy the britax frontier 85 because I would need a convertible.. Please help or if you found another brand of car seats to fit 3 across please let me know..i really appreciate it!!