My baby is 6 weeks old and she only poos every 3 days. Is this normal? The doctor says he is not concerned. I went to the pharmacy and she told me to try a bit of water between feedings, but it still doesn't happen every day.
The frequency of bowel movements covers a wide range - and is about as widely discussed.
My sources (in German, so no use posting a link here) say everything from multiple times a day to once a week is normal. With three days, you should be fine, as confirmed by your pediatrician. Unless your baby appears to be unwell or experience pain when passing stool I wouldn't worry.
The recommendation of your pharmacist may in fact be dangerous - there is a real danger of water intoxication (see here and here) if you give too much (more than 2-3 oz) water in one go or too much in total. Rule of thumb: Water to drink goes together with the introduction of solids, typically with the second or third regular meal per day. And please never prepare the baby bottle with more water than the instructions call for, there are plenty of articles like this one detailing how babies got malnurished and had seizures due to this.