I have tried to have sex three times but came (ejaculated) before penetration. What should I do?

This is due to premature ejaculation. There are home remedies for this before you go to a doctor. Try masturbating an hour or two before sex. This will help you last longer. During sex, focus on distracting thoughts before orgasm and you can also pause briefly and gently squeeze the head of your penis prior to having an orgasm. If neither of these exercises help, visit a doctor.