How to remove one of two linux OSes?

I have two Ubuntu OSes on my laptop and I want to remove one of them. I see by Disks that I have three partitions: Part 1 EFI system mounted, Part 2 Linux filesystem mounted, and Part 3 Linux filesystem not mounted. I guess the unwanted system is on partition 3. Looking around the net there seem to be a lot of flame wars about wiping the disk, deleting partitions outright, using GParted, falling back to Windows. Sheesh! I was foolish enough to install another OS and now I want to remove it rather than re-install Ubuntu (even though I have the install down to 15 minutes.) Most of my frustration has to do with the grub menu and I have tried Grub Customizer but the results are not satisfactory. I would prefer learning how to safely remove an unwanted OS.

Without reading flamewars: for years I had 2 Linuxes installed, the layout was similar:
partition 1 for system A
partition 2 for system B
partition 3 for home and data.
When I used system A for everday usage, system B was for experimenting, and possibly ruining.
When I planned a “distro hop”, I installed the new distro to the system B, and when it got my everyday system, it took the status of system A, so the other partition started to be for the experimental things.
Since I completely moved to Debian, I just feel familiar, comfortable, and I rely on it, I don’t need a secondary system anymore.
Do you feel the same with your current Linux installation?
If yes, just delete the unmounted (currently unused) partition, and then run an update-grub.
(Edit: just thinking a bit more… maybe, it’ll be good to reinstall GRUB from the system you are running:
sudo grub-install /dev/sdX where sdX is your disk that holds GRUB currently)

The boot menu entry for the system was on the deleted partition will vanish.
So you will have an unallocated space on your disk.
Boot up a live image (installer pendrive) and run GParted from there.
Increase the size of your neighboring partitions to take advantage of the disk space.
I’d assign the freed up space to /home, but it’s up to you, how you decide.
You can make more complex partition movements, resizes if it’s needed, if you do such thing, you can do it, not a rocket science. Just be very cautios: if you delete a partition, double check if you really delete which you intend to delete…
Having a backup of all your important data is always the best to have, so if you make a mistake, still you will have your data.

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What @kovacslt says is a good simple way to do it, but it is not the only option.

You can leave the partition as it is, and just remove the filesystem which it contains.
Gparted will do that easily, just format the partition. Again, as @kovacslt says, make sure you reformat the right partition. A reformat will,destroy all information on the partition.

And there is yet another option. You can leave both the partition and its filesystem untouched, and just remove all the files.
To do that, make sure the partition is mounted, cd to the root of the filesystem on that partition, and do rm -r * as root or with sudo. All the files will disappear, but the filesystem will remain. Again, be careful you are at the root of the correct partition.

Before doing anything, it would be a good idea to make a full system backup.