Pregnant from precum after plan b!? !

My boyfriend of i of almost a year are new to sex and we just had sex Last Thursday. We used protection but the condom broke! (Trojan ultra thin condoms) when the condom broke he had not ejaculated all he had done was pre ejaculate. he had peed many times after his last ejaculation taking care of the semen that could be left in the urethra. the condom broke right at the tip of his penis and immediately he pulled out and we went to get option 2 (plan b) this has been the third time we have taken plan b (twice last year and once this year). Should the emergency contraceptive still be effective since we have taken it 3 times? I'm supposed to start my period April 20th (today). I'm having breast tenderness but this is normal for me before my period. Should we be worried about a possible pregnancy?