The Brown Bess and Charleville muskets were both smoothbore arms. Both had many variations and if you were to go to the Royal Armoury in Leeds (England) or Eccole Militaire in Paris (at least when I last visited), you will see many, many pristine examples of both weapons. Besides bore diameter (the French Charleville was slightly smaller), the basic difference in construction was the British practice of usings pins to attach the barrel to the stock as opposed to the French method of using barrel bands. Anthony Darling's little booklet, Red Coat and Brown Bess is an excellent introduction to the Brown Bess. There's a sequel for the Charleville that is also available from the same publisher, Museum Restoration Services in Canada.
Now, the Ferguson was an entirely different gun. Unlike the two muskets discussed above, the Ferguson was a rifled arm. It was unique among rifles in that it was a breechloader. The breechplug was attached to the trigger guard and one full rotation of the trigger guard exposed the breech for loading. You could overload it and rotate the trigger guard up, and then sweep the excess powder into the pan. Only 100 Fergusons were made for enlisted men. Ferguson had an officer's model (it featured chequering around the wrist) for himself. I'm not sure how many were made for the British East India Company, but there were some. Lance Klein wrote an article ("This most barbarous weapon" or something like that) several years ago that appeared in Muzzle Blasts magazine and it may still be available online. Klein said that after Ferguson was injured at Brandywine (actually Chadd's Ford), his unit was disbanded. He did not attribute it to any conspiracy against Ferguson or his gun, but rather that the men, who were drawn from various units, were desperately needed to flush out the thinned ranks. DeWitt Bailey claims (British Military Flintlock Rifles) that some men took their guns with them and points out a painting (Battle of Paoli) as proof. BTW, if you want a Ferguson, be prepared to shuck out $6k for a custom gunmaker to assemble one for you.