Sorry. Now I finally understand what you want to achieve. Hm, at least I guess. You would like to treat your 3 harddisks /dev/sda, /dev/sdb and /dev/sdc as one single disk? Correct?Using LVM to treat 3 disks as one
If this is the case then you will have to use LVM - logical volume manager. LVM can create a volume group which holds a logical volume that in turn holds 3 disks.
I admit I have never used LVM on Ubuntu or Mint, because none of my machines holds more than a single harddisk.
And the last time when I used LVM this was on HP-UX when they still used a GUI named sam. So this was a while ago. And giving you the HP-UX steps, even if I remembered them correctly, would not help.
Therefore I looked up 3 instructions on using LVM on Debian, because LMDE is based on Debian directly. The Ubuntu article might be irrelevant, because LMDE is not based on Ubuntu.
+ Q: How do I install using LVM? Is there any more information about using the LVM installer? LVM installation doesn't work!
+ How to Manage and Use LVM (Logical Volume Management) in Ubuntu
The Ubuntu article suggests that only the alternative installer is able to use LVM during the installation of Ubuntu.
To the best of my knowledge there is no such alternative installer for Mint including LMDE. This might mean that you cannot setup LMDE and make it use LVM from the start, but that you have to setup normally first (without LVM) and add LVM support later on.
It is, however, imaginable that the Ubuntu article is not up-to-date and current installers can make use of LVM during the initial installation. I really do not know.Setting up on /dev/sda initially and extend to 3 disks later
In case using LVM from the start should not be feasible or in case you do not like the idea of fiddling around with it, here is another approach:
Personally I would setup Mint on /dev/sda completely in the first step, including swap and /home and /media/myfiles.
Once the system has been setup completely and is working fine, I would relocate /home to /dev/sdb and modify the file /etc/fstab accordingly, because you want /home to be available at boot time.
Once the system is working fine with this setup, I would relocate /media/myfiles to /dev/sdc and modify the /etc/fstab accordingly, because you want /media/myfiles to be available at boot time.
In order to realize this approach no LVM will be required.
But, when I close gparted I can choose only the /dev/sda partition because it is where I have to put my / directory
GParted will display and modify one physical harddisk at a time.
This means you will have to configure everything that you wish to configure for /dev/sda first and apply.
Next you will have to configure everything that you wish to configure for /dev/sdb and apply.
Finally you will have to configure everything that you wish to configure for /dev/sdc and apply.