You love Kate, Daddy loves me

When Stinker turned three in October, I thought I was pretty lucky to have escaped the terrible twos without too much terrible trouble. Now I just feel foolish. Stinker is going through something. He's throwing tantrums, laying face down in the garage, pounding his fists and kicking his feet. I'm not sure if it's the transition to preschool or the transition to underwear (no more diapers!) or both. But he's not acting like himself. And he's specifically taking it out on me. And it hurts. The other night, Chris was at a work function. I had put Smiley to bed, and I wanted to spend some special time with Stinker. I was trying to cuddle with him, on my lap, and I said "I love you." He replied, "No, you love Kate. Daddy loves me." My heart ached. I responded, "I love you and Kate. And Daddy loves you and Kate too. We both love you both." But I don't know why he said it or why he would feel that way. Is this the start of sibling rivalry? What do I do to turn this around? What's the right thing to say? This is hard!