三者均含有“傻”的意思，但是三者側重點不同。silly尤指像小孩壹樣的可笑的、荒唐的；fool壹般含有愚弄（用來取樂）的含義；stupid則指因腦子不好使而做了蠢事。
以下參考了牛津詞典的釋義：
silly?
adj. ? 1.愚蠢的；不明事理的；沒頭腦的；傻的 showing a lack of thought, understanding or judgement.
2.（尤指像小孩壹樣）可笑的，荒唐的，冒傻氣的 ?stupid or embarrassing, especially in a way that is more typical of a child than an adult.
3.不實用的；鬧著玩的 not practical or serious.
noun ? （常用於向孩子指出其愚蠢行為）傻孩子，淘氣鬼 often used when speaking to children to say that they are not behaving in a sensible way.
fool
noun ?1.[C]?蠢人；傻瓜?a person who you think behaves or speaks in a way that lacks intelligence or good judgement. ?
2.[C]?（舊時國王或王後豢養供人娛樂的）小醜，弄臣?
(in the past) a man employed by a king or queen to entertain people by telling jokes, singing songs, etc.
3.[U,?C]?(BrE)?(usually in compounds 通常構成復合詞)?奶油果泥，蛋奶果泥（甜食）?a cold light dessert (= a sweet dish) made from fruit that is cooked and crushed and mixed with cream or custard
verb ? 1.[VN]?~ sb (into doing sth) 欺騙；愚弄 ?to trick sb into believing sth that is not true.
2.[V]?~ (about/around) (with sth) ?說蠢話，幹傻事（常為逗樂） to say or do stupid or silly things, often in order to make people laugh.?
adj. ?[only before noun]?(informal)?傻的；愚蠢的 showing a lack of intelligence or good judgement.
stupid
adj.1.欠考慮的；糊塗的?showing a lack of thought or good judgement.
2.(disapproving)?(of a person 人)?笨的；傻的；腦子不好使的 ?slow to learn or understand things; not clever or intelligent.
3.[only before noun]?(informal)?（用以加強語氣）討厭的，惱人的?used to emphasize that you are annoyed with sb/sth.
noun ?[sing.]?(informal)?傻子，笨蛋（常用於開玩笑）?