I take 12.5 mg hydrochlorithiazide daily and forgot my meds on business trip will miss 3 days. ... will this be a problem?

Unlikely. You are most likely taking this for high blood pressure and/or fluid retention. Those conditions may not be as well-contolled for the 3 days you are off them but it is very unlikely that this will have any long-term consequences. High blood pressure creates problems when it is high over many years, not just a few days, unless it gets extremely high, but you are on such a low dose this is unlikely.