One way of making it more robust would be make a copy on DVD or BluRay, as I have read that several professionals do, for long term archiving, but this might be seen as too much of a *** for a small/non-professional catalogue.
Is one of the hard drives in a RAID setup? This would increase the safety factor and mean that it would take a three fold failure for pictures to be lost. Additionally, the use of professional quality hard drives built for a hard life in NASs etc,like the WD Red Pro, would give a meaningful increase in life expectancy over consumer level disks.
That part sounds fine to me, but you don't mention making regular and automated backups of your Lightroom catalog file. This must be included in whatever backup plans you have.
I mention in the post that I "regularly back up the latest catalog to all 3 hard drives." Is that what you mean?
you should keep your catalog backups "forever". It may happen that you modify hundreds of images by mistake and recognize it moths or years later. Then, you need your old catalog backup!
I compress them using 7zip which shrinks my 3Gb catalog to 100MB.