This is almost the same setup that I am shooting, with a couple minor differences.

CVA Magbolt 150 .50cal 295 grain powerbelt bullets with aerotip, and 100 (2 pellets) of pyrodex pellets. 1.5-5x bantam scope.

this setup is very solid, consistent 2-3 inch groups at 100 yrds, have taken many deer with it with no problems.

as to the problems you are experiencing. If the pellets are falling apart they are either to old or have at some point in time come into contact with moisture. trash em and get another box.

second of all, If i were you drop the 3 pellets down to 2, it is simply not neccessary to shoot 3 pellets. from what I have heard and read it is very unlikely that the 3rd pellet will even have a chance to burn in such a short barrel and additionally there may be some safety issues with the cva with 3 pyro pellets as they have not been "proofed" for that much powder.

check all the mounts for your scope and use some locktite on all of them to ensure that when you tighten them down they stay tightened.

additionally do not ram/pack the pellets in the bore. simply allow them to seat themself and then place the bullet on top of them with moderate pressure (make sure it seats fully, but no banging the ramrod down etc)

if these actions don't work, you either have a bad gun (it happens, return it and get another one, or you have a bad scope) In order to determine which of these is the problem I would see if it will group without the scope.

Best of luck to you!