you have a number of options on charging multiple batteries
-the simplest/cheapest route would be to buy a parallel charging board (around $15 or less), that you connect all the batteries to and then plug into the charger (charging at 3A would def be necessary for 3 batteries).
tho I'm not too sure how well the stock phantom charger does with parallel charging, since it's so crude and simple.
you might want to consider buying a much more powerful and robust charger to use with a parallel charging board, so you could charge at a higher charging rate (especially if you purchase more batteries).
plus you could use it as a stand alone charger along with your Phantom charger, so right there you can charge two batteries at once.
the cool thing about all the other chargers out there is that they have readouts.. and a buzzer to tell you when it's done charging!
-the most professional route to go would be to buy a fancier new charger that can balance-charge up to 4 different kinds of Lipo's at the same time. - and has a readout showing how each cell is charging, how much voltage is left to go, etc.
if you're serious about this hobby, and have the budget, check one of those out.
-you could also buy additional DJI/Phantom chargers. but while they will work, they're cheap and not the best chargers, nor do they have any functionality other than a few blinking lights.
