1）be to do:
(1)表示事先商定、安排或准备要做的事情。如：
The students are to meet at the school gate tomorrow. 明天学生们将在学校大门口集会。
2）表示可能性，必要、责任、义务、禁止等。如：
Her necklace was not to(couldn't)be found. 她的项链找不到了。 2) be going to do:
1) 表示将要发生的事情或打算最近要进行的动作。如： It is going to rain soon. 快要下雨了。
2) 在含有条件状语从句的主从复合句中，主句一般不用be going to，而常用will（第一人称用shall）。如：
The football match will be put off if it rains tomorrow. 如果明天下雨，足球赛将被推迟举行。
3) be about to do:
1) 表示即将发生的动作，在时间上指最近的将来。如： We are about to start. 我们就要出发了。
The new school year is about to begin. 新学年开学在即。
2) 在含有be about to do的句子中，不能再加时间状语。如： Wrong: The medical team is about to start immediately.
Right: The medical team is about to start. 医疗队就要出发了。
附：be to do 用法的详细讲解：
be to +动词不定式中的be to用作情态习语, 这时的be to do表示: “计划”、“安排”、“义务”、“应该”、“可能”、“命运”等。(have to, ought to)。如: He is to have a holiday. (表示将来)
The committee is to meet today. (表示计划、安排)
You are to go to the hotel where rooms have already been booked for you. 1. 表示 “将”、“计划”、“安排”。(意思接近于be going to) 如： Their daughter is to get married soon.
It was the last film at the cinema，which was to close next day.
was/ were to do 表示过去曾经计划要做的事，或者过去应当做的事，而且从现在的角度来看已经实现了。如：
I felt nervous because I was soon to leave home for the first time. The expedition was to start in a week’s time. was/ were to have done, 表示 “本打算”、“本计划”或“本应当” 做的事而没有做成或没有发生。如:
I was to have seen him last Wednesday, but he didn’t come. We were to have been married last year.
2. 表示 “义务”、“应该” 。(意思接近于should， must， ought to， have to) 如： No one is to leave the building. 谁也不得离开这楼房。
You are not to smoke in this room.=You are not supposed to smoke in this room. You are to be back by 10 o’clock. 你必须十点以前回来。
4. 表示 “命运”, 将来必然要发生的事, 译作 “注定……”。如：
He came to power, but he was to play dearly for it: soon he was assassinated. They said goodbye, little knowing they were never to meet again。
本回答被网友采纳be to do
通常新闻报道中谈论近期即将发生的事件用be to do
例：Police officers are to visit every home in the area.
表示正式或官方的安排、正式的指令或者给予建议，被动式可以让要求或者规定更加客观。
例：Children are not to be left unsupervised in the museum.
必须是人为可控的未来的事件才可以用be to do
例：The President is to return to Brazil later today. （或...will return...）
The disease will appear again.（因为人类不能控制疾病的爆发，不能用disease is to appear again,）
但如果是谈论过去某人在今后遭遇了什么事情或发生什么情况，不管遭遇的事情情况是否人为可控，都可以用be to do
例：Matthew Flinders sailed past Tasmania in 1770, but it was to be a further 30 years before he landed there.
在if引导的从句中，如果主句的事情是if从句的先决条件，则if从句通常用be to do
例：If Lopez is to win gold at the next Olympics, he needs to work on his fitness. （主句先锻炼，从句才能赢冠军）
If Lopez wins gold at the next Olympics, he has said that he will retire from athletics. （Lopez曾说过，如果他赢了下届冠军，他就会退休，从先赢冠军，主再退休，所以从句不能用be to do）
Be about to do
通常在对话中说到马上、很快就要/不要做或发生人为或非人为的事情，用be about to do
例： We are about to eat. Do you want to join us?
Appearing on TV might make her famous, but it's not about to make her rish.