I'm guessing the Dr diagnosed the colic? It is usually worst the first 4 months or so but unusual to start at 3 months (normally it starts earlier), just to be careful not to write something off as colic when it could be something else.

My son had bad colic for the first 4 months, but it really eased off after that. In the meantime we tried different things, we would find sometimes massaging his tummy would help, sometimes a warm bath and soothing music, sucking a pacifier, being rocked, nothing ever worked all the time however and it was mix and match.

We also tried colic drops, but those ddn't really help, they might be worth a try though. If he is formula fed you might want to try a lacto free type just in case that is the culprit, they used to say breastfed babies don't get colic but these days they acknowledge they can and do (mine did!). If you are breastfeeding maybe look at your diet (they say to avoid spicy food).

Here is an article I found online which might be useful:

Colic - NHS Choices

If all else fails remember this will likely get better very soon by itself, they just seem to grow out of it.