How do i treat massive bruise and lump on buttocks after fall down 3 stone steps?

Ice and time: Bruising on the buttocks is a difficult area to treat. Ice packs for 15 minutes every 3-4 hours while awake should help keep the swelling down. Do not apply heat as that could cause more bleeding into the bruise. Most large bruises heal up over a few weeks. If it is not resolving see your doctor.

Bruise: Will usually resolve. However evaluation to rule out fracture. Ice application will be helpful