Not a solution to your current problem, but in case you need to resize mounted btrfs partitions again, this may be useful (except I don't know if there are extra steps to take with LUKS).
It worked for me though (without encryption).
If possible, post output of sudo parted -l /dev/DEVICE and/or sudo gdisk -l /dev/DEVICE (where DEVICE is your boot drive) from the live ISO so the experts have a picture of your partition table.
If you have a backup of /etc/fstab it may be useful too.
I think there is a small chance to salvage it, LUKS stores its encryption header at the beginning of the partition and hopefully those last 3GiB were not in use.