First you create a directory /mnt/home
# mkdir /mnt/home
But then you mount /dev/sda2 to mount, which shadows over /mnt/
# mount /dev/sda2 /mnt
Then try and map to a folder that does not exist as you never made the folder on sda2 which you mapped to /mnt/
# mount /dev/sda3 /mnt/home
Is this intended?
This would work
# mount /dev/sda2 /mnt # mkdir /mnt/home # mount /dev/sda3 /mnt/home
because you mount sda2 to /mnt/ then create the folder on sda2 (Which is now /mnt/) then map /dev/sda3 to this folder (Which exists on /dev/sda2)
The real question is why do you want to map /dev/sda2 to /mnt/? /mnt/ is for mount points, sure you could store then on a different physical drive but it won;t really save space or anything.