now and then与now and again 、once in a while

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。

right now.. 1. 立即，马上 2. 此刻，刚才，现在 right now 此刻，刚才，现在 just now 刚才 in a while 一会儿 at once 立刻

now and then 有时, 时而, 不时 Now and then we heard shots in the woods. 我们不时听到森林里传来的枪声。 now and again 时而, 偶尔, 有时 Drop in now and again. 有空过来玩吧。 时不时, 偶尔, 间或 He went to see them once in a while. 他偶尔去探望他们。