If I have 3 batteries at 35% is it safe to charge one of them in the drone using 2 of them in the charging station from the fly-more combo to get additional flight?

I don't see any problem, they are protected for undercharge.

Hello there. Thank you for reaching out and for these information about the DJI Mavic Mini's intelligent flight battery. Yes you may do this process. In addition, I will post a link where in you can find the Intelligent Flight Battery Safety Guidelines ( https://dl.djicdn.com/downloads/ ... y_Guidelines_EK.pdf ). Thank you.