This Maine Powder House techie has given you an opinion. You might ask him what it is based on. Lyman's Blackpowder Handbook (10th printing August 1992) details tests of G-O 2Fg versus G-O 3Fg in a .54 caliber pressure gun (that is Gearhart Owen powder, now replaced by Goex).
They said:
'Weight for weight the 3F does produce greater velocity and pressure. A comparison of velocities and charge weights proves interesting as illustrated in the following example from the above chart.' (which was tests of 3F and 2F in 20 grain increments from 40 grains to 180 grains, (you read that right) for velocity and pressure).
CHARGE VELOCITY PRESSURE
140 grs. 2f 1779 FPS 8500 LUP
100 grs. 3F 1740 FPS 11700 LUP
____________ ________ _________
40 grs. 39 FPS 3200 LUP
'Nearly equal velocities are obtained through the use of a notably lightrer charge of 3F powder. Pressure, while up, is not at all alarming and this loading will minimize fouling-an importaint benefit to the roundball shooter.
We then decided to use FFFg in all but .58 and .75 caliber during the Rifle and Musket tests. FFg would be used in .58, .75, 12 gauge and 20 gauge.'
Now to the same publication, 2nd edition written by Sam Fadala, published 2001. They again use 3F up through the .54 caliber, Goex, Goex Clear Shot and Elephant brands. (Also Pyrodex P, a 3F equivalent). Same maximum charge of 120 grains. Velocity and pressure comparison for 230 grain roundball in .54 caliber with 120 grains 2F got 1803 fps (32" bbl) with 8,300 PSI and 120 grains of Goex 3F gave 1735 fps with 8,300 PSI. (that's their numbers). And lest you should pucker up over those heavy loads and pressures, this manual shows many loads, especially in .45 and .50 calibers with conicals, with pressures above 20,000 PSI, one I note at the moment that is 33,500 PSI.
I have a friend here, Dean, who made a copy of the old Green River Rifle Works Hawken from their parts (back in the '70's I suppose), .54 caliber. He said he and friends would often use up 7 pounds of 3F black powder over a weekend at his cabin, informal shooting. He uses 80 grains of 3F, and he probably shot 50 to 100 pounds of it through that rifle. I shot it and chronographed it, and the bore is not rusted or pitted. 80 grains of Goex 3F gave 1725 fps, 34 inch barrel. So to anyone who wants to use 3F in a .54 caliber, you might find other things to worry about.
My own open country mule deer load for .54? In my Green River Leman (I made it in their shop in a class) with a 34" .54 Orion switch barrel (from the original .45 caliber), I use a Hornady .530 round ball and 120 grains of Goex 2F at about 1860 fps.