I think I'm looking for a formula or function that takes as inputs the charge level of 3 batteries, and then estimates how long it will take to charge all 3 batteries to 100%. I can get the charge level when I'm connected to the drone. Hmm... As a web developer I imagine I could even provide a free web service that does this calculation for folks.

I would use this function to determine when is the latest time prior to my flight that I need to start charging my batteries. I believe this will also give me the highest possible charge on all 3 batteries, as opposed to charging too early and losing a bit, or too late and not getting a full charge.

And for the ultimate, you put your charger behind a smart switch programmed to go on at just the right time.

Tricky as it would depend on which charger you used.

To fully charge a battery from 1% to 100% (you shouldn't discharge a battery to less than 20% regularly) it will take 1 hour and 30 minutes with the original DJI Charger that comes with the drone,
From 20% to 100% it takes me 1 hour and 10 minutes more or less to charge.
The hub charger that comes with the fly more kit can only charge one battery at a time, and will charge with the same current (relative to time) than the normal charger that comes with the normal kit.
So then you can calculate:
Fully charge the 3 batteries from 1% to 100% will take 4.5 hours.
To charge the 3 batteries from 20% to 100% it will take more or less 3.5 hours.

To charge faster you can buy a 3rd party charger that can charge all of the 3 batteries at once, then it will take 1.5 hours to charge fully, 1 hour more or less to charge from 20%.
However, I Don't recommend using 3rd party chargers, they might damage your batteries. And considering that the batteries cost around $120 in my opinion it is not worth it. I prefer waiting for them to charge than potentially destroying the batteries and for the batteries to light my house on fire in the process.

Chargers are cheep. I have 3 batteries. Got one charger originally and than bought 2 more. So now I can charge all 3 batteries at the same time about a couple of hours before I go out and fly. As I keep the batteries at about 50% or 3,8V  per cell when not flying, it takes me just over one hour to automatically fully charge all 3. This if for the LiPo batteries with 4 cells for Mavic 2 Pro.