Does Deathcap give +3 bonus damage to every spell and ability, including non-damaging ones like Stupefy?

Ruling: Deathcap only adds +3 bonus damage to spells and abilities that already deal damage. Non-damaging abilities like Stupefy do not gain the bonus damage.

Nuances:

• The spell or ability must do damage at the beginning (inherently) to benefit from the bonus damage
• The passive applies to a unit you control, so all damaging effects from that unit get +3 damage