How can I get transit visa from Guangzhou to Hong Kong to attend fair by train and came back?

I already have single entry permit but now I planned to go to hongkong to attend the fair. I came to know that we need a multiple type for that.
I am going to attend the fair by train. Can u pls guide is there any transit type so that I can go to attend fair for 3 days and then came back? If so, how can I apply?