I found running wires for my audio install in my Mazdaspeed 6 to be pretty easy. I assume the 3 is much the same. The door sill trim will hold huge numbers of wires and are only clipped in place, so they come up easily. Likewise, the console comes out pretty easily. There are any number of Mazda forums you can go to to get car-specific help. The guys at the Mazda6Club forums helped me a lot, and at the DIYMobileaudio forums as well.
http://www.diymobileaudio.com/forum/index.php? The most difficult part of my install was getting the speaker wire through the molex connectors in the door frames. That required patience and a steady hand on the drill to not damage any of the other wires. Take your time, do lots of car-specific online research (I found a guy at DIY doing the same install in the same year Speed6 that I wanted to do, so I learned a lot from his generosity).