starting birth control pill late

So my period ended on June 24th and I didn't take my week 1 to week 3 pills because, heck I don't know, I just missed 3 days and thought I shouldn't take it. New to this. Is it okay to take my week 4 pills guessing that my period will come at the end of this week? Or should I just toss the pills? Also, it expires on August 18th. Help a lady out! So new to this!