The transmission fluid should be changed every 30,000 miles regardless of what your Mazda 3 owner's manual says. Changing the transmission fluid will keep your transmission healthy because the fluid lubricates the moving parts of the transmission. Flushing the transmission fluid is a simple job that can be done at home. Flushing the transmission by draining and adding new fluid then repeating the process is better than having the fluid flushed by a machine at an auto mechanic shop.
Slide underneath the car and locate the transmission fluid reservoir. It will be located under the transmission roughly underneath the center console of the interior of the car.
Put a pan underneath the transmission fluid reservoir to catch the draining fluid. Unscrew the drain plug with a socket wrench. The fluid will take about four minutes to fully drain. If the fluid is dark in color then you know the fluid is old and needs to be changed. New fluid is pink in color.
Screw the drain plug back on the transmission fluid reservoir and clean the excess fluid from around the tank with a rag.
Open the hood and remove the dipstick leading to the transmission fluid reservoir.
Put a funnel in the dipstick's location and pour new transmission fluid in the reservoir. Refer to your owner's manual to find out how much fluid you should add.
Replace the dipstick when you are finished and drive the car for at least 20 minutes. Driving the car forces the new fluid into the transmission, and mixes the old fluid, which did not drain, into the new fluid.
Repeat Steps 1 through 6 again. Draining the fluid for a second time will help remove old fluid that did not drain the first time. Draining the fluid twice or three times in a row will flush the transmission and is preferred to having a machine at an auto mechanic's flush the transmission. The machine removes dirt and grime from the walls of the transmission and mixes them into the fluid. The dirt then causes blockages in the transmission.
