The Heavy Armor Master feat (PH p.167) grants the following benefit:
While you are wearing heavy armor, bludgeoning. piercing, and slashing damage that you take from non-magical weapons is reduced by 3.
Recent errata to the Player's Handbook changes the definition of an unarmed strike:
Unarmed strike doesn’t belong on the Weapons table.
Instead of using a weapon to make a melee weapon attack, you can use an unarmed strike: a punch, kick, head-butt, or similar forceful blow (none of which count as weapons). On a hit, an unarmed strike deals bludgeoning damage [...]
Should the damage from an unarmed strike be reduced by Heavy Armor Master?