I haven’t spent a lot of time looking at Dark Souls III specifically, so I can’t speak to it in particular detail. In general, the previous Souls games have had a focus on large movements that make it easier to interpret and interact with what’s happening on screen.
This makes the games play better, but it’s not realistic. There’s two reasons for this. First, actual armed combat is shockingly lethal with little to no warning. As lethal as Dark Souls is, it does not capture just how dangerous these weapons are, when handled correctly. Second, you’re viewing the game from across the room, on a screen. This means the small movements that are critical tells in an actual melee would be impossible to see on screen.
For example, the Broadsword’s move-set in 1 and 2, use wide strike arcs. In real combat, you would never want to do this, because it would tell your opponent exactly where the strike is coming from, and give them time to block it. It would also leave you incredibly open to incoming attacks, which can, and does, happen in Dark Souls.
Generally speaking, in Dark Souls, a weapon’s move set is far more important than the item’s actual stats. There are ways to work around poor base damage, but a bad moveset can’t be fixed. The game groups most weapons into various classes. Straight swords, Thrusting Swords, Halberds, Bows, Whips, Ultra Greatswords… no, that is one of the weapon classes. Weapons in a class will usually share most of their moveset, with a few variations. In Dark Souls the Longsword and Broadsword have a basically identical move set except for one heavy attack. Thing is, this is an entirely artificial system, and has nothing to do with how actual combat works. It makes for interesting game play options, but unless there’s an actual mechanical limitation with a weapon (like the Estoc), it’s not something you’ll see in the real world.
The move sets are also not incredibly representative of how individual weapons are actually wielded. The Lucerne and Estoc come to mind. The Lucerne was a 15th century warhammer with a beak on the reverse edge (for pulling away shields, hooking into riders and dragging them down, and other similar utility functions). In Dark Souls, all of its strikes use the beak. The Estoc was a longsword with a sharpened tip, designed for thrusting. It’s notable because they did not have a sharpened blade. In Dark Souls, they’re distinguished from the Rapier by having a slash attack on their one handed heavy attack. This is notable because it’s one of the few things the real weapon could not do.
The new wrinkle with Dark Souls III is the weapon arts. From what I’ve seen, they’re more in line with the Old Hunter’s version of the Moonlight Greatsword; a charged attack that consumes mana, making it more important for non-magic builds in DS3. The actual moves the system unlocks aren’t any more realistic than what was already in the game.
Dark Souls is still a very interesting setting (as are Demons Souls and Bloodborne‘s), but the combat in those is more, “visually engaging” than realistic.
Incidentally, this is also the case with almost all of Bloodborne’s transformation weapons. They’re there to make the weapons more interesting on a gameplay and visual level. Not because a cane that transforms into a chain whip or a comically massive hammer with a sword in it’s grip, would be a good idea. (That said, the Threaded Cane is stylish as hell.) Bloodborne also gets a decent excuse; the entire game is actually a nightmare/dreamworld; so it’s possible all the weapons you’re finding don’t actually exist in the setting’s real world.
No, Dark Souls isn’t a particularly authentic model of medieval combat. But, it is an entertaining one.