now and then与now and again 、once in a while三者有何区别,请能够举例说明更好

now and then 时而,不时（置于句尾）
e.g.He went to see his grandmother every now and then.
他时而去看看他的祖母.
now and again 时而
e.g.He chops in now and again.
他不时插一两句话.
once in a while 偶尔,有时(置于句尾）
e.g.He comes to see me once in a while.
他现在是偶尔来看看我.