Apologies, I think I got a bit ahead of myself in the description.
Once you’ve determined which partition is which (in your case, /dev/sda1 does appear to be the EFI partition, and /dev/sda3 appears to be your root partition), you need to mount them in this order
mount /dev/sda3 /mnt
mount /dev/sda1 /mnt/boot/efi
Could you send me the output of
lsblk -no FSTYPE /dev/sda3
andll /mnt
?