I am in a relationship with a guy for the past 3 years, and his friend is also in love with me for past two years. This guy knows that am committed with his friend, but still he is in love with me. I really feel sad for him like feeling that I have hurt him badly by avoiding him, feeling so much pity and guilty.

Edited/T

He knows you're with someone else.
He is responsible for his feelings and should work at overcoming them.
No need to feel guilty unless you have encouraged him.

Don't feel sorry for him. Its not your fault he cannot handle the boundaries of good behavior, and get his own girl.

Keep ignoring him, and hope he gets it one day.

It's fine to feel bad for him, but it sounds like he understands that you are dating someone. Don't try to avoid him. Just treat him like another one of your friends because he is isn't he? I think that that is all he wants from you right now. Don't feel awkward with him because all he wants to do is stay in your life in some way. :)