Well, the FN/Mag is gas-operated (like the M60) and the MG3 has a delayed blowback action. What is the ROF of the MG3 these days? I didn't know it was reduced. In my days it was faster than the MG34 and that one was around 900 rpm's. At least that is what the manual said. The MG3: 1000–1200 rpm. Of the two i would choose the '34 simply because of the single-round selector. We also had tucked away some nice tripods for the '34.
Well ,the action of both guns kinda crosses the line. The M240/FNMAG, does use recoil to lock and unlock the toggle bolt assembly along with being gas operated. It has been many years since I really was into all the details of how these weapons operated. And I personally never was able to fire an MG3. I understood the ROF of the MG3 to be 1000rpm , when I was in the US Army in the 1980's. The M240 had a high setting of 950 rpm. And that was a "fire hose" if you held the trigger down and the weapon would get away from you on an unlocked sliding pintle mount on the M1 loaders hatch in a few seconds of full auto .
Fun stuff I must say. Especially when you steal an extra 100-200 rounds from the ammo point, when you were only supposed to get 100, and then repeat the same for 6 tank qualification runs in one day :mil wink: . I CAUSED an ammo shortage for some the night runs of my tank company. They had to go borrow bullets from another range. I hate to think what kinda ammo shortage I could have made with an MG3
Both used the same ammo, so I don't really see a difference tween the two; firing an extra 50 bullets would be extremely problematic, and just increase the chance of a cook-off. And I have seen the results of that from an M240 in a coax mount in an M60A3. They can/will literally "blow up" at about 2000 rounds full auto non stop, if the barrel don't melt first. I saw a staff sergeant once melt two barrels and destroy 2 M240's before the third simply blew up. Dangerous practice but fun.
Too bad I can't recall the # of parts an M240 had when "field-striped" . I think it was "6". Had to be one of the most simple, elegant, designs I have ever seen for such a deadly piece of equipment. I'd like to break down an MG3 and really compare the two. Maybe someday
As to the .50 Cal as an "anti-tank" weapon
, yes they can still cause spall on the turrets of most modern MBT's from the side and rear aspects, and can also damage the engine compartment area's. The best of M2 AP rounds will go through about 3/4 inches of armor and will cause spall with about one of inch of armor at 500m. The 50cal keeps APC's from ever being considered "tanks" on any "real" battlefield, besides the Big Bullets that real tanks can fire , and even true tanks have to respect what a 50cal can do to them.
. It still is a fair anti-tank/armor weapon,93 years later 
....Anti-APC, however....The new Raufoss multi-purpose ammo is nice. Anti-infantry it's overkill. Besides, it is seldom seen on proper tripods like we had these days. Mostly mounted on vehicles with very rudimentary grip/sighting arrangements. The Norwegian Vinghöi company made a very nice double shoulder-stock arrangement for vehicles. Otherwise it's just waste of ammo.