Some people state that on a third date you have to sleep with a guy. That is not true. In fact it is not the best thing to do.
When you are intimiate with someone it is because you really love them. You can't tell how much you love someone by the third date. Love takes time. You also don't know how many STD's this person has or how many people they slept with in the past.
There is no time limit when it comes to having sex with someone. It's all about how much you love each other and if the both of you are comfortable. If a guy keeps pushing and you are not ready then clearly he is after one thing. If he really wants to be with you he will be with you, weather or not you sleep with him on the third date.
Make sure you are in a relationship or have been dating a guy awhile before you rush into sex. Rushing into sex too soon can ruin a relationship. There are also a lot of diseases out there so discuss protection and make sure you and him get tested before you do anything you might regret. One night can have consequences so make sure it's with the right person.