How to mount partition permanently?

Unmount is greyed out because the partition isn't there to be unmounted. You haven't mounted it yet.

To make the partition auto-mount, first find out the UUID of /dev/sda3 by doing:

sudo blkid

I'll assume you're using ext4 on /dev/sda3. Open up gedit like so:

sudo -H gedit /etc/fstab

(Or, in the command line, sudo vim or sudo nano instead of gksudo gedit.)

Now you want to add a line to the bottom of that file. It should look something like this (UUID will be different):

UUID=03ec5dd3-45c0-4f95-a363-61ff321a09ff /media/works ext4 defaults  0      2

For more info on fstab, this is a good resource.

Use Disks utility. Select the disk, then click Additional partition options icon. Choose Edit Mount Options from drop-down menu.