Iv had a cold for th past 3 days and jus developed a dry cough..been taking a vitamin c aswell as a cold medicene but the cough is stil persistant?

Not that uncommon: A cough from e.g. Bronchitis may last several weeks so it is not surprising to hear of a cough lasting 3 or 4 days. Keeping your overall health strong helps. Might get some benefit from medications and possibly inhalers, check with your doc.

Common thing: Have you had a flu shot? Pneumovax? Sounds like a common thing. Antimicrobials and additional testing does not seem useful at this time. Over the counter meds and "halls" may be useful. Tea and chicken soup, as our grand mothers told us.