What's the best way to deal 3rd degree bed sore?

Surg cleanse & care: Wound care with debridement of necrotic tissue as well as wounds care with adequate nutrition, minimizing soiling, pressure relief and a clean moist hydro active environment have been shown to be important.

Pressure relief.: Bed sores are pressure sores (decubiti) and nothing will work unless offloading (pressure relief) is carried out. Then good wound care, removal of the nonviable (dead) tissue, and healing, grafting, or muscle flap surgery by a plastic surgeon may be necessary.