While humorous and funny are similar, silly means more foolish, or lacking common sense. You can watch a silly cartoon but you may also scold soemone for being silly.

Out of the three of these words, "humorous" is the most bookish. "Funny" is a generic term that can be applied to anything that makes you laugh or even something that confused you/went against your expectations. The latter is usually used with a touch of sarcasm. For example: 1. "That movie was really funny!" (i.e. it made me laugh) 2. "She is really funny!" (i.e. she has a good sense of humour and makes me/others laugh). 3. "I found it a bit funny that there was no chicken at KFC." (i.e. you thought the irony of the situation was, well, humorous). Silly is a bit more casual, and it can have positive or negative meanings depending on how it's used. "She's so silly!" is probably positive, and means that she is kind of carefree or acts like a child. "Stop being silly!!" is negative, and it means that the person should be more serious. More abstract: "I think it's silly that some Americans are still fighting against gun control." (i.e. I think it's dumb/stupid, it doesn't make sense to me). ((PS - I'm American, I can say that ;)))