A replacement handguard on the Benelli M3 improves your forward hand position and may add M-Lok slots for a hand stop or barricade stop. The M3 is a dual-mode shotgun -- selectable between semi-auto (inertia) and pump by rotating the forend -- and its handguard must be compatible with that switching mechanism. A replacement handguard gives you a better surface while maintaining the pump/semi functionality that defines the M3.
Confirm M3-specific forend compatibility on the individual product page -- the dual-mode mechanism is unique to the M3 and not all handguards accommodate it.