Well, the answers depend on the muzzle velocity you're getting and the ballistic coefficient of the projectile you're using. The muzzle velocity, of course, depends on a variety of factors and the only way to be sure about it is to measure it. Do you know anyone who has a chronograph?

No...the reason I'm trying to find out is this:

They (manufacturer video) say rifle is rated for 150gr Pyrodex or 3 pellets. However in the video and manual that came with it in the loading portion, they say use 2 Pyrodex pellets or 100gr. I'm trying to find out what is the effective range for deer using 2 triple 7 pellets, is there a valid reason for not using 3.

150 grains would put it in the Magnum class.
Is it rated as a Magnum? If not I wouldn't use 150 grains.....well mabe pellets I might, but not free flowing powder. I have heard that pellets dont burn quite as fast and as completely as powder.
100 grains is a good load for most ML's, as going higher gives erratic shot groups.