There are different thoughts to the rule of three when it comes to different gear.

Shelter for example, no one wants to carry a second tent, let alone a third. But a poncho and some para cord can make an impromptu tent/lean to. And if worse comes to worse, those XL trash bags can be used to water proof your sleeping bag, or as a makeshift poncho.
Or a wikiup liner to keep the snow and rain off.

Same goes for water purification, no one is going to carry three filters, but a steripen along with a handkerchief and some polar pure tablets cover all the bases.

Fire? Lighter, matches and ferro rod (metal match).

Knives? Now here’s my downfall.
Over the last quarter century my choices changed daily sometimes.

Regardless of what I have on me, my kit has a small Rapala filet knife in it, it has become the knife I use for 90% of my hunting needs.

I’m picking up some Mora “puukko’s” for the wife and kids, inexpensive and if they lose or break one, I won’t shed a tear unlike my custom knives.

I probably carry too many sharp pointy things, but as the camp butcher, I always needed another sharp knife and every body else’s were guaranteed to be dull. When you’re carving up 2-3 deer or elk a day for two weeks, sharp knives are vital.

Clothing? I keep a seasonal bag next to my BOB, as well as another bag with some good clothes, in case we evacuate and I don’t or can’t wear my woodland clothes.

Extra socks and underwear? And then some. Wearing the same pants and jacket for days on end is one thing, but clean dry socks?

3 GPS? One GPS, two compasses, one map, one map sketch with distances, notes and bearings.

The rule of three is not just three of everything, but three ways to do or accomplish things.