Is using half a pill of finasteride in alternative days 3 days a week effective? Most people consume one pill daily.

It can be effective, if used as directed. However, I suggest seeing a hair loss expert now to evaluate and go over your condition and provide you information about your best treatment options. There are great non-invasive options like prp/progesterone and/or hair transplantation. See an expert. Best, Dr. Emer

I agree with Dr Emer that you should seek the advice of an expert as there are a number of treatments which he has alluded to which may be of more permanent benefit to you.Best. Dr Mandraccia

This dose is likely more effective than not using finasteride at all .... but is less effective than 1 mg daily. 

Reducing the dose of finasteride is ok, but it will be less mildly less effective then the full dose of the drug