All three have the same meaning. The difference is in their usage. 'Yes' is standard English and to be used when you are responding formally, in a mail or speaking to someone. 'Yeah' and 'Yep' are both informal and can be casually used when you are speaking or writing to friends, family etc. Just to let you know, most dictionaries say that 'Yep' was first used in the years between 1830-40 and 'Yeah' was first used in the years 1900-05. Cheers.

"Yes" is formal. "Yeah" is a little more informal. "Yep" is something very informal. This is typically used between (young?) friends, I believe. I don't think that it is appropriate to use it in a formal situation.

same as No, Nah, Nope

语气和态度不同，就像汉语中的“是的”、“是啊/对啊”、“对头”这种感觉~意思一样，但是用起来的话需要分场合