This came froma review byRandyWakeman:
Though not recommended by Hodgdon, most inline manufacturers currently allow and promote the use of "three pellet loads" for velocities far in excess of what loose black powder or Pyrodex can possibly achieve. How fast? One load out of the 2004 Knight catalog shows a muzzle velocity of 2417 fps, another is 2639 fps! These velocities can actually be bettered in a longer barreled gun, like the Thompson/Center Omega.
Triple Seven pellets, used in three-pellet configuration, actually produce a bit more muzzle velocity. Where Pyrodex pellets are pressed into shape from black powder and Pyrodex RS, Triple Seven pellets are made from straight Triple Seven, and are harder to ignite. For what it is worth, I've personally found Pyrodex pellet loads to be more consistent, and more accurate.
A word of caution is in order. Hodgdon does not condone the use of more than 100 grains of their Pyrodex or Triple Seven pellets in .45 or .50 caliber muzzleloaders, and they clearly, loudly proclaim that warning with every box of pellets sold. Several people have asked, "What pressures do three pellets produce?" Well, there is no finite answer. Much is necessarily contingent on projectile weight, type, and caliber of rifle.
http://www.chuckhawks.com/difference_black_powders.htm