Is it bad to give a 3 month old baby water?

Absolutely, as long as the water is clean and uncontaminated. If there is any chance that the water is contaminated with anything, boil the water first and then let it cool down before you share it with the baby. ANY water coming from a lake, creek, river, etc. has the potential to be contaminated with chemicals and/or diseases. In many communities throughout the world, even tap water may be contaminated. Clean clear water is good for babies. Just remember, water doesn't provide any nourishment.