For Garmin, just format as many cards as you need (per their guidance, I don't have the specifics in front of me).
Once you create the first one and it completes in Garmin Pilot, then eject it and insert the new one. I then just start the process over in Garmin Pilot and it creates another card.
I do all 3 screens at the same time this way.
[I use a Mac, not sure if it differs on a PC]