While battery hub is charging all 3 batteries one by one in it self, should I take them out one by one or is it ok to leave the fully charged one in the hub when its charging the next battery ? I have mavic mini 2 battery hub

Leave them in.  The only reasons to take a completed one out is to either go flying and use it, or if you have more than three batteries to charge.

Hi there. Thank you for reaching out. The explanation of K6CCC on post #2 is correct. You can just leave the batteries on the hub when you charging it until it will all fully charge, you can take 1 or 2 fully charged batteries if you want to use them or change it and charge a new set of batteries. Thank you for your continued support!

Hi there. Thank you for reaching out. The explanation of K6CCC on post #2 is correct.

Thank you.