You don't have to match the key to the drive. You have to match the key to the motherboard. So just flash one firmware with one key to one drive and figure out which 360 it works with and leave it in that one and flash the other.
But now since you told me that. You actually have to match the drive to the correct console now.
Easiest way is to open the firmware file in jungleflasher. See what drive the firmware belongs to. Flash it to that drive. Then put it in the xbox if it does work put it in the next and so on. You will have to do it with all 3.