Hii

#1 is wrong. #2 is a way to ask someone to help you. #3 is a way for you to offer help to someone else. You are having trouble yourself. You need help. You want to ask someone for help. You would say "Can you help me with this?" You see someone having trouble. They need help. You want to offer them help. You would say "Let me help you with that." You could also say "Allow me to help you with that." "Let me" and "allow me" are polite forms. You are asking permission. In US culture, helping someone is breaking into their social space. Consider giving someone physical help, like lifting them. You say "let me" or "allow me" to show that you respect their space, and won't break into it until you have their permission.