The Heroism spell says(PHB, 250):
Until the spell ends, the creature is immune to being frightened and gains temporary hit points equal to your spellcasting ability modifier at the start of each of its turns. When the spell ends, the target loses any remaining temporary hit points from this spell.
So, does that mean that after two rounds and not taking damage the "hero" will now have +6 HP if my spellcasting ability modifier is +3? Or does it just keep refilling to +3?