i tryed having sex three times and i couldnt erect,is it becaouse of mastubation

Likely. Men who masturbate frequently can often get this problem. The body is used to a much more instant gratification and has difficulty getting erect during normal intercourse.

I suggest you stop masturbating, or at least wean yourself down, cutting down the amount significantly. The problem isn't likely to go away unless you retrain the body to react normally to sexual intercourse.

It could also just be anxiety.

If you're afraid of something, getting naked in front of a girl, not performing well, getting her pregnant, etc. it can prevent you from getting erect.

Masturbation could be a part of it too. If you hold off for a few days, especially if you masturbate often, your body will be looking for a release and you'll get more easily aroused.