(old-ish advice, I could be wrong)
LRMs: If you have someone good with a targeting laser, these are incredible at dealing significant damage to a specific section. If not, they're great for crowd control and picking off targets that are engaged or fleeing. Keep in mind that they have a minimum range of 180(?) meters.
PPC: A stalker can fit four PPCs on it and still be effective, or six and be a heat machine. If you alpha strike just right, you can two-shot an enemy mech.
GR: Vs. the PPC, it weighs more and generates less heat, and doesn't leave a trail that's as visible as the PPC's.