Yes, they can have many of the medications we take, but unless you have a gram scale and a basic knowledge of what they should and shouldn't have I would not recommend that you give anything unless directed to by a vet. At three days old i'm gonna guess that they weigh between 30 and 70 grams, which is almost impossible to dose correctly with human drugs.
Sounds like you have figured this out already, but ducklings without their mothers are not water proof and cannot have access to water they can stand in or they will get soaking wet and hypothermic. This is true even on very warm days. The waterer is use for mine is one of those quart sized ones with a red base.
-Kathy