If you're not going to put plastic on the walls you could run a cross flow booth where you have just as much air coming in as going out. But if you're going to use plastic to cover the walls (I suggest you do if you don't want them painted too) then just keep more coming in than going out. You could use one fan sucking air out at your walk in door with the other two at the garage door pushing in. That will work. Make sure you use decent filters and let the fans run for an hour or two before you clean the inside of the booth. This will let any loose filter material to be removed before you paint. After you let the fans run a while, clean the inside of the booth the best you can to get any dust out before you bring the car in to paint.
Good luck with your paint job.
"When buying a used car, punch the buttons on the radio. If all the stations are rock and roll, there's a good chance the transmission is shot." - Larry Lujack