I have a 2006 Mazda 5. I have had it since 2007, when my son was born. We currently have a 5 year old and a 3 year old. That 3rd row is pretty tiny. We occasionally have to cram kids friends back there... it can work, but it is extremely cramped. It will not fit a rear facing child seat and probably would not fit a full size child car seat. We can fit a booster back there (for kids over 30 pounds), but there is very little leg room. Our 5 year old definitely would not fit back there in his Britax... we can fit them back there in the boosters though. The Mazda 5 really is not a minivan... it is a hatchback with sliding doors.
Bottom line, we really like the car, but we only have 2 kids. With 3, the kid in the back is going to be in one of those booster seats where they use the adult seat belt, not a full size child seat. And it is quite hard to climb back there, so the kid in the back will need to be able to get himself in and out of the seat.
EDIT: I missed that you said you already had 3 kids... that is going to be very difficult in the MAzda 5 IMO. That 3rd row is not really a full sized seat and the middle seats are going to be right up against your seats in the front.