I fell 3 days ago my knee still hurts and a little swollen what should I do?

Get evaluated. Problem could just be a contusion where you have bruised the soft tissue in front of your knee. Contusions usually don't hurt inside the knee but outside. If it is a ligament injury or a problem inside the knee it will usually hurt when you move it. If your knee is locking (can't straighten it out) or pain on weightbearing see doc sooner than later.