Look at this this way: It's fine for your son to prefer Daddy. Why? Because you and Daddy are on the *same side*. You're team members! You need to want your son to love Daddy just as Daddy needs to want his son to love you. Actually, your son loves you both.
The minds of three-year-olds are a little bit different from grown-up minds. They form very strong preferences, but those preferences change from time to time. Just as he may insist on having strawberry ice cream *every time* ice cream is offered, so he insists on having Daddy there when he can get him. It's strong right now, and it's going to change.
What can you do? Try to see the humor in it - that will help you view the situation realistically. Encourage your husband and your son to do things together, just as if you planned it that way. Above all, of course, you want to love your son's daddy. Tell him, "I'm so glad our boy has a dad like you to be obsessed with!" What your boy needs most is a mommy and daddy who actually love and encourage and back up one another. But you know that.