Is augmentin 875mg safe in 3rd trimester of pregnancy for sinus infection?

Yes: By the 3rd trimester, fetal development is much less an issue and thus the Augmentin should be safe. If your congestion is from the sinus infection, it will improve in 3-4 days. However if the problem is from allergy, a nasal steroid may work. Vasomotor rhinitis of pregnancy from hormonal changes can also be an issue in which case a trial with nasal antihistamine may work (not cat. B however)