mount /dev/sda3 /mnt
mount /proc     /mnt/proc
mount /dev      /mnt/dev
mount /dev/pts  /mnt/dev/pts

You need to use -B to bind mount the pseudo filesystems, and probably also need to include /sys

grub2-install /dev/sda3
grub2-mkconfig > /boot/grub2/grub.cfg
exit  # out of chroot?

reboot

Should work?