I assume they're trying to go for some weapon-triangle balancing factor for the three LMG-type weapons.
The Minigun is the highest potential DPS with its high mag size and firing rate, but it forces you to sacrifice movement and requires trigger control to earn that DPS.
The Stoner is the in-between for mobility and firepower; easier to use than the Minigun but not as powerful.
The Shredder is less powerful than the others but gives you extra anti-trash control in exchange for less damage per bullet. Ultimately you're hitting similar breakpoints except for maybe Scrakes and Fleshpounds.
LMGs are just in an unenviable spot in general in this game.
Commando wasn't originally designed with LMGs in mind and was intended as a less-severe trash-oriented Sharpshooter that required headshots, trigger discipline, and managing reloads with mid-size magazines and average reloading...all of which are design choices that don't really jive with high-caliber guns that have enormous magazines. So when the Stoner was finally meme'd into existence, TWI erred on the safe side and made it pretty underwhelming because a hard-hitting fast-firing gun with 80+ rounds would be kind of OP. (The FAL exists, yes, but even that one requires lots of recoil management and has a fairly small mag and ammo reserve.) The long reload times are meant to counterbalance the Commando's damage combined with the higher-than-average magazine size.
Same deal with the Minigun, and so the Shredder also follows suit. All three have received buffs over the game's lifetime because the assault rifles are just better in the hands of skilled players. They're perfectly viable and the Minigun is good memes, but there's better options.
I'm just trying to make the 3 big machine guns unique, but guess you can call it a weapon triangle balancing.
Though you have the purpose of the Stoner and Shredder swapped. Likely because I made that typo that called the Shredder the MG3 "Stoner" (not sure why you deleted the MG3 part in my sentence). This is what I wanted to do with the 3 guns.
Minigun is the largest magazine with highest damage per magazine and potential DPS, but the hardest to aim and control.
MG3 Shredder is basically the M60 from Far Cry 5. It has the highest damage per shot (I'm thinking at least 40 is ideal) at the cost of the slowest fire rate (again no higher than 800 RPM). Though I did the numbers and it's DPS will actually just be slightly better than it's current form. It's magazine size is the middle of the road between the minigun and the Stoner. It's more designed for precision gunners (since it has the best iron sights of the 3 guns) who prefers to conserve ammo as oppose to firing wildly like the minigun but still feels very much like a machine gun. Also comes with the highest purchase price out of the gate (thinking 2500 dosh).
Stoner is the weakest with the smallest ammo because it's free, but also the cheapest to buy out of the gate. It's compensated by being the only one of the 3 that you can upgrade if you don't want it lagging behind the other 2 by that much. However obviously this makes the gun 10 weight which reduces your loadout options.
This is how I like my machine guns and it creates good options for Commando players.
AR-15 and Bullpup - the starting guns
Sentinel - the backup buddy/gunner for players who are bad at watching their backs and/or just needs extra firepower
Tommy Gun, AK-12 and HMtech-401 - the mid-tier weapons with the option to turn the Tommy Gun into a hard hitting LMG or the AK-12 into a high DPS weapon.
Carbine Rifle, Scar and Fal - The hard hitting rifles
Famas, M16 and 501 Grenade Rifle - the guns with the hard hitting alt fire
Stoner, MG3 and minigun - the big heavy machine guns with large magazines that takes long to reload. For players like myself who loves the "high capacity mags" and "eat lead" perk skills.
On another note, I'm one of those players who don't like using the Fal on Commando. Sure it has a high primary fire, but it sacrifices WAY too much for that 70 damage (too heavy, too expensive, magazine too small, ammo pool too small) and these characteristics are more noticeable on 6 player games. I find the Scar can do the same job and I get way more mileage from it too. If I want to hit hard as a Commando, I often find pairing any combination of minigun, sentinel, Famas, M16 and medic grenade rifle the best way to go. The minigun + sentinel combo or the two grenade rifle combos has not failed me once yet. I've killed more large zeds with those combos then I thought was even possible with a Commando.
EDIT: Though I do admit I want the MG3 altered that way because I want it to be able to rival the minigun in a sense and join my personal top 6 Commando weapons. It's pretty trash in it's current state and I legit am thinking of testing a 9 weight Tommy Gun against the base Stoner and MG3. The 2500 dosh MG3 version I'm thinking of will pair up nicely with weapons like Scar, M16, Famas and sentinel.