Thanks for your question.
Common colds in a 3-month old baby would require urgent evaluation and treatment by a paediatrician if it comes with poor feeding, high fevers, eye discharge, or greenish nasal discharge.
However, if these are absent in your baby, you can employ home remedies. First, considering your child's age, its safe to avoid any kind of over-the-counter drugs for treating colds in older children.
To relieve the symptoms of colds in a 3-month old, employ the following home remedies:
1. Get an over-the-counter nasal saline drops which help to thin the mucus in the nostrils,
2. Sunction the mucus in your baby's nostrils with a bulb syringe which must be keep clean.
3. Use a humidifier in your child's bedroom to moisturize the air and moisten the dry and inflamed nasal linings and reduce cold symptoms.
4. You should increase your baby's feeding with breast milk as a way of keeping him hydrated. Do not give water.
5. You could sit in a steamy bathtub with your baby for a couple of minutes before bedtime. The heat helps to thin out the mucus reducing the nasal congestion.
If these do not resolve the symptoms after a few days, please visit a paediatrician for evaluation.
I hope I have answered your question and I hope your baby gets better soon.
Best wishes.