All of these convey the future tense, but there are slight differences between each of them. 1. I will rest for two days. (This is a simple statement of the speaker's intention) 2. I will be resting for two days. (This is in the future continuous tense, showing that an action starts in the future and continues through a given period into the future) 3. I'm going to rest for two days. ( "be going to" is a phrase used mainly in conversation. It means the same as "will," so both #1 and #3 mean the same thing) I hope my explanation makes this clearer. If you still have questions, let me know.

There is a difference between "will" and "going to", and this is taught at elementary level. When we are talking about our own plans, "will" signifies the original idea - we don't know how, when or where we will do the action, but the idea is in our head. After we think about it for some time, we have a kind of plan. This is when we use "going to".

1 and 2 convey the future span. Number 3, conveys it could start right then or a little later in the future (For example: I'm going to rest for two days after I finish this assignment)

I think all of them are OK. But I am waiting a native English speaker to answer it.