We have been going out for the past 3 yrs. And one day he asked for sex, but I said no. But he keeps on asking and I am afraid I might lose him or he might go get it somewhere else. I am scared cause I love him.

NEVER give sex to supposedly get love or to keep a guy around. It doesn't work that way, and will actually work just the opposite.

NO one should engage in sexual intercourse until they are prepared to have a child. So you ask him whether he wants to be a father.

No form of birth control is 100% effective, so engaging in sexual intercourse carries the risk of pregnancy.

And I agree with WG's comment. If he is pressuring you, tell him to stop. Tell him you do not want to engage in sex until you are ready. If this is a make or break thing with him, better to know now.

Fear of losing someone is the wrong reason to have sex. If you have to be pressured into it, you are not ready.
Should you do it? No!

Think properly and ask all relevant questions to him about the consequences. What happens if you got pregnant and he walks away? Don't give yourself and your future away easily!

No, if he really loves you, he will not leave you just because you do not have sex. If you want to wait till after college or wait till marriage, that is what you tell him, or just say NO I am not ready.

If you lose him for this, he did not love you anyway.

Uhmph... he asked, asks and will keep asking... its quite natural... but asking for it is one thing insisting and trying to pressure you into doing it its another...

Just the fact that you have doubts and ask "should i do it?" is reason enough for NO as an answer... The fact that you would want to do that to keep him around is a second reason for a NO, simply won't work (as you have already been told)... If you tell him you aren't ready he should get it and give you more time. And I suppose you're talking about intercourse...
And if he is still so immature to not give you time and go somewhere else then he wasn't that great of a guy to begin with, at 21 he should know better, I know that it doesn't sound good at your age but with time you will understand it

Sex is not about making people happy and too stop them from leaving you, it's about love. Does he love you? Even then sex has its other consequences like possibly children are you ready for that?