You can replace the bottle and then reset and prime whichever of the 3 you want manually in the settings for the Trident. The automated Tasks make for an easier way to do A or A/B/C only, because that’s how Neptune wants you to use the reagents. You can calibrate whenever you want, regardless of what you are changing.
The Alk test uses A. The Ca and Mg tests both use B and C. So B/C are always used together, but A is completely different- A used up relatively faster if your alk is high, slower if alk is low. I asked Neptune to make a task to replace B/C for obvious reasons, but they said no and cited mainly that they are sold as a set as opposed to a science/engineering reason. I still hope they will at some point, it is kind of annoying as is.
I change B/C when either is out. I change A when it is out. When I change a bottle, I don’t keep reagents from the old to mix with the new, because there is very little to be gained by it and these do have expiration dates. I also don’t refill A at the same time as B/C unless they happen to run out at the same time by chance. For me, with an alk of about 8.5 dKh, my A lasts quite a bit longer than my B/C.
At some point here I’m going to have to find someone with high Alk doing the same so I can sell my extra A and/or buy their extra B/C.