will be 和will be doing 和be going to do 三者区别.

will be 和will be doing 和be going to do 三者区别.
我昨天看了一下新概念二的13课,里面有几个句型都是will be doing将来进行时的结构.如:
At present,they are visiting all parts of the country.
They will be arriving here tomorrow.
They will be coming by train and most of the young people in the town will be meeting them at the station.
Tomorrow evening they will be singing at the Workers' Club.
The Greenwood Boys will be staying for five days.
但是,课文尾处两个句子表达的意思是差不多的,却用了一般将来时.
During this time,they will give five performances.(这个为什么要用一般将来时,而不是将来进行时.)
As usual,the police will have a difficult time.They will be trying to keep order.
而且,它们能和be going to do互换吗?
它们什么情况下可以换?