You need to keep them spinning on a regular basis, and replace them as they begin to fail.
This is usually a precursor to SMART errors happening in the near future, but unfortunately, it can still result in corrupted replication and corrupted backups; as your backups would be backing up the rotten (corrupt) data.
I've witnessed this happen on both Seagate and WD drives, on systems with ECC memory. I can only suspect this is due to HDD manufacturers wanting to reduce their error rates, and RMA rates: it may happen with the ECC bits in a sector is corrupt, making bitrot undetectable. Instead of giving an error (and being grounds for a RMA replacement), the HDD firmware may choose to return non-integrity-checked data; which would usually be correct but also could be corrupt.
It's why filesystems like ZFS and btrfs are so important.
My rough estimation of this, based on my own experiences and those on r/DataHoarder, suggests 1 hardware sector (4KB for most drives post 2011) will silently corrupt per 10TB year. Such corruption can be detected via checksumming disc systems like ZFS.
Usually, the whole sector is garbage, which is not indicative of cosmic ray bitflips.
External flash memory storage like USB sticks and SD cards fare far worse. In my own experience, silent corruption occurs more like 1 file per device, per 2-3 years; irrespective of the size of memory. I've had USB sticks and SD cards return bogus data without errors, so often. I only know because I checksum everything, otherwise I would have thought the artefacts in my videos or photos came with the source.
If, in 2020, you are not using ZFS or btrfs for long term archival, you are doing something wrong.
ext4, NTFS, APFS, etc may be tried and tested, but they have no checksumming, and that is a problem.
However, at work, I have backed up ~200TB of data to a large server with RAID-6 and ext4, storing the backups as large .tar files with par2 checksums and recovery data, and regularly scrubbing the par2 data. I have yet to see any corruption whatsoever. These are enterprise-grade hard drives. This is the strongest evidence I have yet seen that the enterprise-grade drives are actually better than the consumer-grade ones, rather than just being re-badged.
I should really get around to converting the main drive to btrfs, but this works well.
Much like other commenters I'm no expert on the topic, but I think you'd have to be pretty incredibly unlucky to have a mechanical failure on 3 drives at once from lack of use, especially if they were from different manufacturing batches.