Totally normal, mama! A few of us went through the same thing when we switched to mixed feeding, ang digestive system lang ng babies ay nag-aadjust pa sa formula, kaya it can take a few days before they poop again. As long as she's peeing well and acting happy, she's okay, but since she's only 3 months pa lang, I'd still mention it to her pedia just to be sure everything's adjusting smoothly, especially with the feeding change.