First off. Do not buy m4 add on. I have heard many negative reviews about it. So please don't buy it unless you want to have fun with m4.
M3 is a slow but very powerful shotgun. It does lots of damage in one blast but rate of fire and freeze rate is long. Making m3 better as a secondary weapon for burst damage or finishing gun. Shotgun on the other hand does less damage but has more spread. It has slightly less freeze rate and more fire rate than m3. While it can hit enemies at long range more easily, the damage is considerably weak at long range. It is a cheap shotgun (by price) good for some situations. Hard to use a spar gun.
PBP Shotgun is the deadly gun people talk about. Small freeze for a shotgun (still has lots of freeze), good rate of fire for it's damage, and deadly damage at point blank range. 4 damage per bullet. (I think)