Hello. my 3 month old baby is extremely constipated. exclusively on formula and only has a bowel movement once every 2-3 days and straining alot?

Soft or hard ?: If the kid is passing pebbles it is extracting too much water from the poop which is what the colon does. An ounce of prune juice 1-4 times a day (spread out) should get things moving in a few days. If the poop is soft, the "strain" may just be the effort a kid using flabby belly muscles & pushing from the breathing muscle needs to poop. They don't have gravity to help.