Affect: This is a term used to describe observable behavior that represents the expression of a subjectively experienced feeling state (emotion).
Mood: This refers to an emotional state that may influence one's perception and behavior. Unlike emotions which are typically triggered by specific events, moods are not necessarily tied to specific experiences.
Feeling: This is the subjective experience of emotion but not all feelings result from emotions. For example, you might feel tired after a long day at work which isn't an emotion but rather a physical sensation.