Text (TE+errata): At the beginning of your upkeep, if you have three or fewer cards in hand, ~this~ deals 1 damage to target opponent. [Oracle 1999/07/21]
It checks the number of cards in your hand at the beginning of upkeep and does not trigger at all if you have more than three. It checks again on resolution and does not deal damage unless you still have 3 or fewer cards. [D'Angelo 1999/06/01]
You target one opposing player each time the ability is played. [bethmo 1998/02/27]