Knowing that the equilibrium concentration of CO2 in water exposed to the atmosphere is "about 3 ppm" doesn't really let us accurately determine how much CO2 we have in the water when we add CO2 to our tank. "About 3 ppm" might be 2.2 to 3.8, for example. A pH drop of 1.0, means we increased the amount of CO2 in the water by a factor of 10 (10 to the change in pH power) But that would mean our water has 22 to 38 ppm of CO2 - not very accurate. But, we can't measure pH much more accurately than +/- 0.2, so our 1 pH drop might be as little at 0.8 or as much as 1.2, and 10 to those powers is 6.3 and 15.8, so the real range of CO2 concentrations is 6.3 x 2.2 = 14ppm to 15.8 x 3.8 =60ppm which is now a useless measurement. Granted, you can possibly do a good enough job calibrating your pH probe so the accuracy is better than +/-0.2, but you will always have an error range greater than zero.
I think it is best to just assume we can't measure the ppm of CO2 in the water. What we can do is use a drop checker to tell us that we do have a significant amount of added CO2 in the water (we aren't just leaking the CO2 or degassing it rapidly). Then we can slightly increase the bubble rate of the CO2 and watch the fish and plants. We can repeat that until either the plants don't grow any better with the last change, or the fish are showing too much misery from the CO2. Back off the bubble rate to the previous rate and that is the optimum amount of CO2 for your tank, with your plant load, your lights, your oxygenation of the tank water, your fish, etc.