I would empty the radiator and refill with a mix of 50/50 water/coolant and do this every 2 years regardless of mileage and you will have no problems.
Draining the engine block as well as the radiator is easy, but then you have to fill the block as well as the radiator or you could risk cracking the block as the cylinders heat up quickly on engine start.
It can also cause the thermostat to stick in some cases because of air trapped underneath it causing the engine to over heat.
I would only suggest draining the complete system if it's gunked up and not as regular maintenance.
Drain and refill the radiator every 2 years and you will be fine. I have done this for hundreds of thousands of kilometers without issue and in most cases without ever having to replace a radiator and/or water pumps so I know this works.
And be responsible, drain the coolant from the radiator into a catch pan and put it in a container and take the recycling depot attached to your local garbage drop off place. (One at Nebo just south of Rymal)
The drain and fill is very easy on Mazda 3. Once you remove the engine undercover, you don't have to jack the car up to access the radiator drain plug, also it drains half of the system, the system holds 8 qts of coolant and radiator drains 4qts.
I did a poor man's flush by draining the radiator and refilling it with distilled water, running the engine until the thermostat opened and draining that again. I did that 5 times, at the end water was clear, so I drained the radiator and filled with one gallon of full strength coolant, that gave me a 50/50 mix.
I did a full flush because mazda coolant is regular green, but with no silicates. Most stores carry dex clones or some type of long life coolant, so I did not want to mix formulas. I put in G-05, and from now on I will do radiator drain and fill every 2 to 3 years, no flushes unless I can no longer find g-05 and will have to change formulas again.