A lot of the online formulas and calculators are in g/l, which I find inconvenient, so I did a conversion. Besides when we break things down, it's simple math.
1 lb sugar = 2.25 cups, which raises the SG of 1 gallon by 0.045
Simple division indicates: 1/2 cup sugar raises the SG of 1 gallon by 0.010 (1/4 cup = 0.005 is more granular if needed)
You want to raise 6 gallons by 0.020:
(0.020/0.010) * 0.5 cup * 6 gallons = 6 cups (3 lbs is 6.75 cups)
I recommend adding half the sugar, stir very well, and check SG. The must should be ~1.080. If the SG differs by much, adjust the amount of remaining sugar added. As much as the sugar addition should be simple math, natural products are involved so results can vary. As mentioned above, removing the sugar is far trickier than adding ....