It's mostly fluff kept for sentimental value. Worst case scenario (complete data loss) would be annoying, but I can deal with it.

That's one of the two things the 3-2-1 rule of thumb doesn't address - depending on the value of the data, you need more backups, or the backup might be overkill. (The other is what you're talking with smeg about, the reliability of each storage device in question.)

I do have an internal hard disk drive (coincidentally 2TB)*; theoretically I could store a third copy of the backup there, it's just ~15GiB of data anyway. However:

• HDDs tend to be a bit less reliable than flash memory. Specially given the stick and SSD are relatively new, but the HDD is a bit older
• since the stick is powered ~once a month (as I check if the backup needs to be updated), and I do a diff of the most important bits of the data, bit rot is not an issue
• those sticks tend to fail more from usage than from old age.
• Any failure affecting my computer as a while would affect both the HDD and the SSD, so the odds of dependent failure are not negligible.
• I tend to accumulate a lot of junk in my HDD (like 490GiB of anime and shit like this), since I use it for my home LAN

That makes the benefit of a potential new backup in the HDD fairly low, in comparison with the bother (i.e. labour and opportunity cost) of keeping yet another backup.

*I don't recall how much I paid for it, but checking local hardware sites a new one would be 475 reals. Or roughly 75 euros... meh, if buying a new HDD might as well use it to increase my LAN.