원래 is quite different from 보통 and 주로.
원래 means "originally; primarily; by nature"
보통 and 주로 both mean "usually"
In other words,
보통 means "generally, or normally"
주로 means "mainly, In a manner of serving as a basis or center"
In short,
보통 is used for a matter that happens a little more frequently than normal. (probability:50~60%)
주로 is used for a matter that happens much more frequently than normal. (probability:80~90%)