Is it safe to take diphenhydramine (benadryl), glucophage (metformin) and lisinopril?

You mean together? Yes, for persons with healthy kidneys. Doctor follow up is needed anyways since these persons must have diabetes and hypertension. If kidneys are diseased, Lisinopril may transiently decrease kidney function. If so, metformin may need to be stopped. This is another reason for need for doctors supervision. Sometimes people miss doc's appointment not aware of these complexities.