The correct answer is you should make a starter. For liquid yeast it is good practice to make a starter for every beer. That way you know the yeast is healthy and you have plenty.
However, I assume you are a relative beginner and have never made a starter. With the beer in the fermenter, it is not the right time to learn.
Just pitch the pack. You will be under-pitching, and depending on how fresh the pack is, you could be under-pitching by a lot. It will make beer. It will probably take longer to start than you are used to; maybe 24 or 48 hours.
Chances are you will notice nothing, but some potential problems with under-pitching are; higher ester production, long lag time before starting, and higher final gravity.
FYI: 3 points gravity difference is basically nothing.