Giving medicine to a 3 month old is never easy so it’s to be a little forceful but with the right technique. Make sure he’s always sitting upright and never lying down. You can try giving the medicine with a strong flavoured syrup like chocolate, maple etc. If it’s a liquid medicine, you can use a syringe without the needle.
it happened to my son too. what i did abit extreme but it works, it's up to you if you are comfortable trying it though. once i pushed in the medicine, i'll force close his mouth and he'll swallow but he'll be crying. medicine helps him so bopian, i need to be rough