You have a chain. I 100% guarantee it. If you try to replace a timing belt, you'll find there isn't one. Don't worry about replacing it unless you encounter problems. If you want to verify what I'm telling you, take off the valve cover and look. Don't forget to replace the gasket when you put it back on. Or you can take the cheap route and look in a service manual.
But as a Mazda technician working with these motors every day, I guarantee you--Mazda quit using timing belts a little while ago. All Mazda 3s, 6s, speed 3s, speed 6s, MX-5s ('06 and later), MPVs (with 3.0 V6 engines only), CX-7s, CX-9s, and Tributes all use timing chains.
Let that "ahh....low maintenance" feeling sink in. =>