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.