They are all correct but they carry different context. ex. 1: We will have a lesson on Friday, then we'll have another lesson two days later. ex. 2: After two more days, we will have a lesson. (this one sounds weird) ex. 3 We'll have a lesson in two days. The third one sounds the most natural and conveys the message best.

An e-mail to my nonexistent assistant

Good evening everybody,

I came across a writing exercise on a Cambridge book and I said "why not give it a shot", so here I am... I'm not supposed to have written more than 40 words, but I wonder whether it's even possible to say all that in so few words. Anyways, please correct my mistakes, I'll be quite grateful to you from doing so.

Seb.

Dear Assistant,

I am writing to let you know I was invited to an engineering exhibition in Frankfurt. I know we have a very busy week ahead, but I couldn’t decline the invitation, for exhibitions like this represent an unbeatable opportunity to promote our services and to make some good business contacts. I’ll be away for the whole week, so it goes without saying that you will be in charge of the company during this time. I know you know better than anyone else that we hold a follow-up meeting every last Monday of the month, but it can’t hurt to <strike>remind it to you</strike>  remind you of it. Of course I expect you not only to attend it, but to inform me of everything that is said in it. Also, don’t forget that we will receive a visit from our partners in China. They didn’t let me know whether they were going to arrive on Wednesday or on Thursday, so please be sure to be present in the office during these two days.

I’m confident that you will do great.