Reduce toughness and cast dmg spell - cause of death?
If you reduce a creature's toughness from 6 to 3 you just treat it as a creature of toughness 3. So dealing 3 damage to it can be regenerated. The 3 damage does not reduce the toughness to 0. It will still be a creature with toughness 3 with 3 marked damage on it that will usually die but can be saved with regeneration.