Spare carseat recommendation

Hi ladies! I'm looking for recommendations for car seats for our other car. I have three year old twins and one on the way. The car seats we have are in my car and if my husband ever needs to pick up the kids he has to come to my work and exchange cars, which has started to get really annoying with all the extra driving. So we are looking into getting 3 Convertible seats for his pickup. Looking for something cheap, since they won't be used everyday, and reliable, so it can grow grow with the kids as the years pass. Any recommendations?