J.A.
Is your 3 year old still small enough to use a 5pt harness booster? perhaps one like this that goes to 65 lbs: http://www.amazon.com/dp/B0011URFRE/?m=ATVPDKIKX0DER Then you should be able to use the lap belt safely to hold him in the miggle and the baby can go behind the passenger, the 6 year old behind the driver. Just a suggestion is all- I don't know what you can or can't afford, but this seems like the least expensive, safe option. Hope that helps! ~J.