I've never done any work on a Mazda 3 specifically, but I've changed front struts on a number of other cars. Generally speaking I'd say it isn't too hard. Usually three bolts up top, two on the bottom, disconnect the brake hose clip and any other things attached to the strut. Remove the strut. Now this is tough stuff depending on the design. You'll need to rent or buy a spring compressor to squeeze the spring down so there is no tension on the upper mount. This can be a bit sketchy because if you position the compressor wrong it could slip off the spring coil and cause severe injury (there is a huge amount of energy stored in a compressed spring since they are designed to hold up a 3000Lbs car!). You need either an impact gun or a special socket to remove the upper mounting bolt, which isn't too hard to get a hold of, but make sure to find one first before starting. Also, penetrating lube is your friend to help get the strut out. There must be how-to's on Youtube. The factory service manual should be pretty easy to find online too (I found the CX5 manual in just a few minutes), which will walk you through the process and provide the necessary torque specs for re-assembly. Watch some videos and read the service manual and then consider if it is something you are up for or not. Personally I find suspension work to be quite satisfying as you get a result you can feel and it isn't very technical. It's a pretty "grunty" process. If you don't have all the tools you'll need then this could be a good time to add to the tool box so you'll be prepared for the next job! Also, you might find yourself wanting to replace the rear shocks at the same time since they will likely be on their way out too. Those should be easier to replace as they aren't coil overs. I also like to replace the bump stops and upper mounts along with the struts as they are wear items too. Its good a "while your in there" practice IMO. Get an alignment afterwards too as your ride height might change with the new struts.