Having unprotected sex after taking oral contraceptives for only 3 days does not protect you against unwanted pregnancies. In Fact, it is always recommended by doctors to use a backup method like condoms for the first 7 days ( I often reccomend 1 month) after starting your contraceptives.
This would increase your chances of pregnancy; however it would mostly depend in which day of your cycle you had sex. Certain days of your cycle are more fertile than others, usually the middle.
I would suggest you to keep taking your contraceptive pills, after this cycle you will be protected. I would consider the use of Plan B to prevent a pregnancy and do a home pregnancy test in approx 14 days.
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