One thing you have to remember is that British Line soldiers who had to accomplish the "three shots per minute" rate were doing so using paper cartridges loaded with undersized balls (.69 cal. for .75 musket) and measured powder. they carried them in a cartridge box hung from their left shoulder across the chest to rest at the right hip. The US and French forces had the same arrangement but with .62 cal balls for the French .69 muskets, which both the US and French troops used.
Reason for the undersized ball was so they could continually load and fire without the need to swab the barrel to clean it out. If they used a normal sized ball such as a .74 ball for their .75 musket, they would be lucky to get two shots off before needing to swab about the barrel so the next charge could be seated. Below is a picture of an actual musket cartridge from the Rev War:
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They were kept in cartridge boxes such as this, typically holding from 18 to 24 pre-loaded cartridges:
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Here's how it was used:
• With the musket under their arm, the cock at half-cock and the pan open, the first thing they'd do is grab the cartridge from the box and flip the top flap up with their thumb as they brought it to their mouth.
• They'd then bite off the top close to the powder, charge, and close the pan. Then they would move the musket to their left side with butt slightly above or touching the ground and pour the rest of the powder down the barrel.
• Then flip the cartridge around and insert the ball part of the cartridge first, crumpling the paper of the cartridge down on top of it so the ball didn't just roll out if the barrel was pointed down. They would push it down slightly in the barrel with their thumb.
• Next they remove the ramrod, ram the ball and paper down on top of the powder, and return the rammer to the pipes.
• At this point, if firing in volleys, they would return their musket to the "Poise firelocks position". American forces would either Shoulder Firelocks or Poise Firelocks whichever the commander had previously specified. This way the commander could see when all troops were ready for the next volley.
• Finally the command would be to "Make Ready" (cock pulled to full-cock position), "Present" - musket shouldered and pointed or aimed at opposing line ("Take Aim" was the command for the American forces) - , and then "Fire".
The British musket troops were not allowed to take part in the line if they couldn't do this 3 times in one minute and in 1778 that became a requisite for the American Forces also.
Because these smoothbore muskets were loaded with undersized lead balls they were not very accurate but they were easy to load. But they didn't need to be accurate. When fired in volleys at you, it was like a giant shotgun with .69 caliber pellets filling the air. At about 50-yards distance the first volleys would hit a few folks. At 40-yards, a fair amount of folks were hit although your shot might hit the man next to the man you were aiming at. At 30-yards, virtually all shots would hit and cause massive injuries to the opposition. So it was vitally important to be able to load and fire more quickly than the opposition.
Ideally you would fire within 30 yards causing massive injuries and often momentarily shocking the opposing line. The the British typically lowered their muskets with bayonets forward and charged the enemy line. Point was to travel that last 25-yards or so before the enemy could reload, and the British were the best at it. So firing quickly and filling the air with shot was the primary point and accuracy was not really a concern for either side.
Now both the Brits and the Americans had rifle troops. The Brits had the Hessian Jaegers with their short, stout and accurate rifles, and the Americans had their rifle companies equipped with the troops personal longrifles. These rifles were capable of great accuracy out to 250-yards or more with their open, iron sights. But they used tightly patched lead balls that were all but impossible to fire that quickly. So they were most effectively used at picking off officers, artillery crews, and troops between about 100 and 250-yards. When musket troops got within about 100-yards of the rifle troops the rifles would typically head back behind the first line of musket troops or head out to the flanks finding cover in the woods to continue sniping at British troops. They could cause great damage to the British, especially to the officers, but battles were really decided by the musket troops.
I have personally seen some folks fire a rifle at and hit a target at close to 3 shots per minute, but not quite. That was typically at a Biathlon with a king of the hill competition where the shooter was allowed to use a loading block and stand his ramrod up in the snow in front of him. The target was at 25-yards for the first round and 50-yards at the second round and if you missed the target, the shot didn't count. To my knowledge, here are no extant loading blocks from the 1770's, nor any primary documentation of their use. It would be safe to say a rifleman would be hard pressed to load and fire two shots, let alone three, accurately with a tightly patched ball (such as a .490 ball in a 50-cal.) within 1 minute - especially if you had to accurately hit your target at 100-yards or more and then disappear in the woods before the advancing Brits got close enough to hit you.
