这三个短语的用法和含义各不相同：
1.What if表示"倘若……将会怎样 即使……又有什么关系呢 ";如果…怎么办
(用于疑问句句首，尤用于询问不希望看到的事发生时的结果)
例如:
What if it rains while we are out
倘若我们外出遇上下雨,那怎么办呢
What if he was going to die!
如果他快要死了该怎么办？
What if relations between you and your neighbour have reached deadlock, and their behaviour is still unacceptable?
如果你和你的邻居已闹僵了，而他们的行为还是令人难以接受，那怎么办？
《柯林斯高阶英汉双解学习词典》
What if anything should happen to the child?
万一这孩子出了差错怎么办？
《汉英大词典》
'What if I had said no?''About the apartment, you mean?'
“我要是说了不呢？”“你是说公寓那件事吗？”
What if he got a guilty conscience and brought it back?
要是他觉得内疚，把东西拿回来了怎么办？
What if Spain reacts to the similar economic pressures which are appearing over there?
西班牙要是对类似于那里正在出现的经济压力做出反应该怎么办？
But what if the government takes no heed?
但如果政府不理会该怎么办呢？
《柯林斯高阶英汉双解学习词典》
What if it rained and then froze all through those months?
要是那几个月一直又下雨又冰冻怎么办？
What if this doesn't work out?
如果这个不行怎么办？
So what if the colours clashed?
那么，要是色彩不搭怎么办？
《柯林斯高阶英汉双解学习词典》
What if it rains?; Suppose it rains?
要是下雨怎么办？
《汉英大词典》
'What if they gave a war and nobody came?' was one of his generation's favored aphorisms.
“如果他们宣战却无人应战该怎么办？”是他那一代人钟爱的格言。
《柯林斯高阶英汉双解学习词典》
其实就是：what if +正常语序的(一个或多个)句子+？即表……怎么办？
What if the train is late?
火车要是晚点会怎么样呢？
What if she forgets to bring it?
要是她忘记带来，会怎么样呢？
2.what of 表示"……的情况怎样 ……又有什么重要性呢 ;
(用于引出与之前内容相关的新的话题或观点)那么…怎么样
What of it 那有什么了不起呢 (那又怎么样呢 )
Well, and what of it (It has no importance.)
嗯,那又怎么样呢 (怎么,那有什么了不起 )
And what of the effect on U.S domestic opinion?
对美国国内的舆论有什么影响？
Well, so much for the producers. But what of the con-sumers?
好吧，关于生产商就讲这么多，那关于消费者呢？
'You're talking to yourself.' — 'Well, what of it?'
“你在自言自语。”“对啊，不行吗？”
《柯林斯高阶英汉双解学习词典》
'This is Hollywood, U.S.A., where they make all the movies, remember.' — 'What of it?'
“记住，这是美国的好莱坞，所有那些影片都是在这里制作的。”——“那又如何？”
《柯林斯高阶英汉双解学习词典》
3. what for为何目的、 为什么 。
What for did you ask that silly question Now he's going to speak for another ten minutes. 你干吗要问那个愚蠢的问题 这一来他又得说上十分钟.
What did you do that for？你做那件事有何目的？
What for? Your English is as good as mine.
为什么呢？你的英文跟我的一样好。
What can I do for you?
我能帮你什么忙？
What's for breakfast?
早餐吃什么？
《柯林斯高阶英汉双解学习词典》
另外，what for还有：惩罚，责骂
get/give sb what for（受到）严惩；（被）痛打一顿，申斥一顿
If he got fresh, two or three of us would pitch on him and give him what-for.如果他挑新的，我们中间就会有两三个人要找他麻烦，把他大骂一顿。
I'll give you what for if you don't tidy your room.
要是你不整理好你的房间,我会责骂你的.
I'll give her what for if she does that again.
她要是再这样干我就收拾她.