Sound advice about getting some protection, pregnancy or a STD can happen so easily.

And please do not blame yourself. You didn't say no this once, but it does not mean its your fault. You've been saying no for 3 years, that's pretty awesome, esp as peer pressure is strong and can be so hard to keep saying no.
If you can try and remember how low you've felt since if and when another man pressures you, it may make things a little easier to say no when you're on a high.

Think of it as experience, and please don't beat yourself up - we all learn by our mistakes, sexual or otherwise, and I think everyone, at any age, keeps on learning!