Robbie,
The tablet you are wondering about could be any number of medications. Gemini is right. Your best bet would be to ask a pharmacist. What ever you do, don't take it.
~Tuck
If it ONLY has #3 on one side and nothing on the other, white and round, according to drugs.com, it is Nitroglycerin .3mg - however, it would be best to double check with your pharmacist - they'd be able to tell you for sure what it is.