Your preschooler is also noticing his opposite-sex parent in a new way, as part of what psychologists call the Oedipal phase. Based on the Greek myth about Oedipus (a man who inadvertently kills his father and marries his mother), the term describes the period between age 3 and 6 when a child becomes romantically attached to the parent of the opposite sex.
A boy's love for his mother, for instance, evolves beyond dependent adoration into pure infatuation: Suddenly, he wants you all to himself. He may even start giving passionate kisses, propose marriage, or say things like "Mommy and I are eating supper, Dad; why don't you go up to your room?" or "We don't need Daddy here right now, do we, Mom?" or "If I shut this door, then Daddy won't be able to get into the house." (A girl's romantic feelings will be aimed at the father, of course, and Mommy will become the target for elimination.)
Strange as it may seem, this phase of development is both natural and necessary. It helps preschoolers achieve a firmer sense of sexual identity and acquire healthy feelings toward the opposite sex. But it can generate some painful emotions as well.
As a child's romantic love for one parent (and rivalry with the other) grows, so do her feelings of guilt, anxiety, and fear. A girl who loves and admires her mother, for example, but also feels jealous and resentful toward her can't help but experience some pangs of guilt and disloyalty. These emotions are then intensified by fear -- fear that the mother will find out what the daughter is thinking and strike back to claim her husband.
Because adults are so much bigger and stronger than children are, the idea of parental revenge is terrifying to a 3-year-old and likely to be buried in the subconscious. But it will eventually come to the surface, usually in the form of bad dreams or fear of the dark, death, monsters, bad guys, and witches. Unless such fears become excessive or a child's behavior toward a parent becomes extremely antagonistic, tolerance is probably the best response.
Don't pretend you don't love your spouse, and don't avoid natural expressions of affection just to save your child's feelings; he needs to see where your loyalties lie. In fact, if he thinks there really is a chance that he'll beat his father out and claim your love, he'll feel even more confused and terrified. It's far better for him to witness a healthy relationship between husband and wife -- one that he can hope to emulate someday.