OP, at that millage most people who want reliability will also replace the tensioner assembly ( spring and idler pulley with new bearings ) when they do the serpentine belt. Personally I would also replace the water-pump because new belts and new tensioners produce the maximum correct tension and the old belt and old tensioner may be producing much less tension and the old water-pump may not be able to handle the new tension very long. And if you are going to open the cooling system to do the pump, you might as well do the thermostat and all new antifreeze.
At around100 K miles many vehicles require other items such as cam chain(s) and cam drive gears, and spark-plugs, valve adjustment check, PCV valve, coolant thermostat, radiator fluid change and maybe a few other items. I do not know if your 2006 mazda 3 with the 2.3l would require all of these items but if it does if you are going to do them, the added work of the serpentine belt and tensioner assembly would be something that would not add that much additional work to the job.
BTW, a couple of years ago Gates came out with a small plastic tool to check serpentine belts with that they gave away for free. All you had to do was ask for it online and they sent them to you for free. Now they have a new small plastic tool that is even better and easier to tell if a serpentine belt is good or bad with. However I do not see any place on there site to get one. But there are videos on the internet about it and it is available for less then $10.00 it still might be available for free from Gates or at a much lower price from them. I just found out about the new tool today when I looked for a link for the old tool to see if you could still get one for free. You might send Gates an e-mail or phone them next week.
Also, a good general rule with the much better material they are made of these days is to check the serpentine belt every once in a while after 60 K miles, and do it even if it looks good at 90 K miles.
Back in the early days of serpentine belts like 1990 they were only good for somewhere between 25 K miles or a little more.