Ah, the Mazda 3. Love the zoom, dislike the back seat width (I call it "not three butts wide"). The ages and sizes of your kids will help us help you.
Some general Mazda 3 notes: my three across is with one adult seat belt, and a RF radian is difficult for the front passenger if he or she is not short, even with the angle adjuster. But, I've had some success by adding in a Coccoro. A FF radian next to a RF coccoro fits fine. Two FF radians next to each other is a no-go, but a RF radian next to a FF radian might work, so if you have a set-up so that you could get the RF radian in the middle (it can't touch the passenger seat, and it won't fit at a newborn angle with my drivers), maybe three radians would work. If your RF rider is a newborn, I'd look at a coccoro over a Radian.
There's some more Mazda 3 three across photos here:
http://www.car-seat.org/showthread.php?t=210811&highlight=mazda