You need to talk to your boyfriend dear. Sit him down and really tell him, make him listen about how insecure you feel. If he is a "great" guy, he will listen to you.
Yes, boys are "allowed" to look at porn and other girls. You can't stop them. But if it is making you feel insecure and sad, then there is a problem. I went through the same thing with my BF too. I snooped and found sites and pictures. It broke my heart.
Maybe suggest fun dates, to get him away from the computer? Spend more time together.
Oh and try not to snoop, I know it's hard. All us girls do it, but if you are snooping, that means you think he is hiding something from you. A relationship should be about trust.
But yes, keep in mind, guys look at girls. It is normal. There is polite ways of looking at girls, and there is leering and practically drooling. And that's rude. So if your BF is making you uncomfortable when you are out together and checks out other girls right in front of you, let him know how it makes you feel. There is a thing called respect.
And don't compare yourself to those cheap girls online. I think it's disgusting that young girls, or any woman puts themselves out there on the internet. You are much more classier then that
Tell him that you are a living, breathing girl and can give him LOVE. That is something a stupid web-cam girl cannnot give. You have been together for 3 years and that's wonderful.
Don't beat yourself up girl, take pride in yourself. Pamper yourself and make sure you treat YOU good. If you don't like yourself, find something that makes you feel good. A new outfit, a haircut, a hobby, or hit the gym (bring your BF!) Working out together is a great way to bond. Remind your guy, what made him fall in love with you in the first place
I'm sure you know how!!
I don't know if my advice is good, but I hope you can take something positive from it.
I hope you can figure out everything! Good Luck!!
xo Niko