The old school way of loading a 3-gun shotgun - now called the "Traditional Weak-hand Load 4" technique, has more applicability to a real world scenario, in my opinion, because it doesn't involve a full or partial dismount of the gun. Of course, it's only applicable if you have shells on your body. I actually don't load 4, I load 2, which is less fumbling and results in less dropped shells. Pluck two out, index, shunk-shunk into the tube. You can use two-shell caddies, 4-shell caddies, your pocket, etc. The mechanics of this approach are pretty basic and the take away is brass needs to be indexed in the hand, regardless of where you carry the shells. If the gun is empty and you grab two shells in your hand, you can port load one and tube load the second, with practice this is a very straight forward approach and works for all tube-fed shotguns and for topping the gun off, behind cover, etc.
You can also see in this video the old school "loop"-style match savers, where it required you to pluck a shell out and put it in the chamber, as opposed to push it with a little bump, into the chamber. Slug-selects from this type of carrier always made a lot of sense to me, allowing the side saddle to hold buck, while still giving you two slugs for easy access. But of course, this format really only works with semis.
~Sigh~ 3-Gun shotgunning used to be closer to actual martial use applications.