little一般带点人情味,表示数量和年龄等,比如the little cute girl
small一般是描述无生命的物体,比如the small tree.再比如The first tree is smaller than the second tree.
tiny是说一个东西非常非常小,有生命的物体和无生命的物体上都可以用,但通常不用在名词前面修饰,比如：it is a kind of tiny cell.这个就是错的.
正确的例子有：Cells are so tiny that you can not see./They are very tiny!