Is It Safe And Effective To Take Plan B Post 3 Days Of Having Unsafe Sex?

Hey! Saturday at 5am I was having sex and me and my partners condom broke. He said he ejaculated inside me so Tuesday I went to the clinic ...They gave me a pregnancy test which came out negative and they gave me a plan B pill..usually the pill make you a little sick but it felt like I never even took a pill. My friends are saying that is to late to take plan b and it is to soon to take a test! HELP?!!