Is it ok to take metronidazole, nitrofurantoin and amox-clav together?

If your Dr. OK's it. Combining multiple antibiotics can have potentially serious complications, including superinfection by bacteria such as 'clostridium dificile'. These three medicines should only be taken together if your doctor has prescribed them go some reason. Never take prescription medicine intended for another person. Strictly speaking, there are no interactions between these meds. (source: epocrates).
Maybe. These 3 antibiotics do not adversely interact with each other. However, one or more of them may interact with meds you regularly take or cause complications in chronic conditions you may have. Unless your doctor specifically prescribed this combination, you should not take extra medicines. And your doctor should be notified of all meds (non-prescription too) you take.