You can download a free copy of the factory service (or shop) manual off the internet for the Gen1 Mazda 3. It shows you where all the mounts are, how to remove them, and how to install them.
The oft-broken passenger side mount is on the front of the engine right above the timing cover and support the engine. The drivers side mount is underneath the battery tray and supports the tranny. The third mount is underneath the car pretty much centered between the wheels, but behind the engine/tranny (like underneath the shifter).