同意曹老师观点

(1)主语+do so (so表示/替代前面提到某个事情/某个行为/某个风俗/某个习惯）

He produced his railway ticket when asked to do so. 

当验票时,他出示了火车票.

It is the custom of foreigners to do so. 

这种做法是外国人的习惯.

It is very impolite of you to do so to customers. 

你这样做对顾客太不客气了.

(2)do it(指代前面或者上文提到具体某件事情，语气没有that强烈。）

I wasn't expecting you to do it for free. 

我并没有指望你白干。

They took a vote and decided not to do it. 

他们进行了表决，决定放弃。

"All right, Mrs Bates," she said. "We'll do it your way." 

“好吧，贝茨夫人，”她说，“我们就按你说的办。”

Whatever you want to chat about, we'll do it tonight. 

不管你想聊些什么，我们今晚聊个够。

(3)do that (指代前面/上文提到某件具体事情，语气要比it更强烈。）

You don't need to do that," said Pearl assertively. 

“你不需要那么做，”珀尔斩钉截铁地说道。

I think he's an absolute stinker to do that to her. 

我认为他那样对待她真太缺德了。

"Don't do that," he says in his soft, accented voice. 

“别那么做，”他带着口音轻声细气地说道。

 Okay then let me ask how you do that. 

好，那我来问问你怎么做的。

备注：英语是一门不喜欢重复，而喜欢十分简洁的语言，因此出现大量代词，代词就解决了意思又表达清楚了而且避免重复罗嗦的问题发生。