MG3 has a heavier barrel to decrease the amount of time needed to change a barrel
Watercooling is heavier weight, yes, but carrying a bunch of spare barrels neglected that. And a barrel can be lost, and a barrel costs more to produce then a jacket containing water and oil
The wehrmacht already requested heavier barrels with the MG42 from day one because it simply overheatted way to fast...
So stationary wise, something wich still happend alot in WW2, a watercooled MG was still much better then the MG42.
And the russian maxim issent that all to bad in moving it to a new position..well.....for a watercooled MG that is....
The gun itself dint weighted that much more then the MG42...But the carriage it came with...
13KG for the gun...
10KG for the gunshield
...40KG for the carriage
Things improved alot with the SG43 however. well, weight wise (41KG compared to 60+KG for PM1910)
But again, i have always been talking about the advantages STATIONARY wise
Even the germans re-used old MG 08's in bunkers on the eastfront because you could just keep them firing and firing and firing and firing.
Never forget what the maxim did in WW1. 10 vickers MG of the 100th machine gun corps firing over 1 million rounds with 100 barrels for 12 hours without one single breakdown.........