My daughter wouldn't poop everyday. I was lucky if it was every other even. She was completely formula fed but I did try to breast feed for awhile but never produce enough and finally my supply went away, But from day one, when she was on all breast milk, she wouldn't poop everyday. If she's not on a regular schedule like every other day, or even every three days it may be caused by changing her diet back and forth from breast to formula. But I wouldn't say it's strictly the formula that causing it.
If she's not uncomfortable then she's ok. Signs of discomfort are gurgling tummy noises (but not right after she eats. Don't mistake them for normal digestion noises). Spitting up more than usual. Kicking her feet up at her tummy the minute you lay her down (she might do this anyways if she's an energetic child like mine was but if she doesn't quit after a bit it might be a clue). And of course crying non stop.
When my daughter was little I payed so much attention to all her functions it was embarrassing. My husband and I still cheer when she poops sometimes. This is normal just remember babies are different than you and me. Liquid diets wether breast or bottle create different types of responses. And not all babies are the same. When mine when on solid foods she started pooping mostly everyday but she still skips a day now and then. I say listen to your doctor and don't worry about it. And stop giving her the juice unless her doctor tells you too. At three months her digestive system isn't ready to process fruit juices. She might not be constipated and this might just upset her natural nutritional balance. What you might be seeing as results of the juice, pooping, could be just the juice running through her at record pace because she can't digest it.
Good luck to you.