HDD installation... am I doing it right?

Ok. Where you are confused is that usually you do not do a factory reset to make your drives "nas ready". A second point of confusion is on RAID. By default, you will be using XRAID. You will have a single data volume, and your files are automatically spread across all the installed disks (invisibly to you). The NAS will also protect you from a single drive failure (allowing you to replace a failed drive with a new one, with no data loss). Note that RAID does not protect against all types of failures, so it is still important to back up data that you think is critical or hard to replace.

So here's what you should do:

(a) first shut down the NAS, and remove the two "old" hard drives.
(b) then install only the "new" hard drive, and power it back up.

At this point, you will see your copied movies.

If you are not running 6.1.6 firmware, then update it now.

From here you have two basic choices - one which uses your 2 TB drive, and another which doesn't.

Option 1) - skip the 2TB drive, and get a new 3 TB drive later...
To do this, you hot-insert one of the "old" drives into the RN104. This means "put it into the NAS while the NAS is running". It should detect the insertion, and add it to the existing 3 TB array. You won't see any increase in size, the second drive is used for protection against failure.

After the NAS tells you that it has finished adding the drive, then hot-insert the second "old" drive. This one will increase your space to 6 TB (reported as 5.4 TiB on the Web UI).

Option 2) - reuse the 2 TB drive also.
To do this, first install one of the 3TB drives back in the PC, and format it for NTFS. Then copy back all the movies to the 3 TB drive.

Then power down the NAS and insert the remaining drives into the NAS ( 2TB, and 2 3 TB drives).
Do a new factory reset

When that completes, you should have about 5 TB of space (4.5 TiB reported in the web ui)

Now copy the movies back onto the NAS again.

Then remove the 3TB drive from the PC, and hot-insert it into the NAS. That should expand your volume size to 8 TB (7.3 TiB reported on the web ui).

BTW, if you do feel the need to prepare an existing drive for use in the NAS, the easiest way is simply unformat it.
There are two ways to do that. One is to use the vendor diagnostics (seatools or WDC lifeguard) to zero the drive.

The second way (in windows) is to right-click on the computer icon, and select "manage". A window will open, scroll down to the disk management section. Right click on the volume of the drive you want to unformat, and you will see an option to delete it.