Hi! I'm expecting baby number three in November and really am hoping to keep my current vehicle - new car seats are far less expensive than a new car

I have a 2005 Ford Focus SW. My daughter is 7 years old, but bitty - only about 43 pounds, 46 inches tall; my son is 4 years old (5 in October) and about 41 pounds, 44 inches tall. They are both currently in Cosco Summit
convertible booster seats (daughter using car shoulder harness, son still using 5 point harness) but I'm assuming I'll need to replace both of these because they have extremely wide bases and leave very little room in between for a new
infant car seat. The center position does have a shoulder belt, not just a lap belt. There are tethers for all three seat positions, but I believe there is NO
LATCH position at the center seat. My husband measured the rear seat and figures we have about 51 inches to work with. Thanks in advance for any assistance you might be able to give. I'm not so much concerned about the cost of the car seats as I am safety and simply fitting them all in!!!