over the last three weeks Or for the last three weeks
over the last three weeks Or for the last three weeks
Which one is correct?
And is it also fine to write 'past' instead of 'last'?. For example, I have been busy over the past(last) three weeks.
What if I want to talk about future? For example, I will be busy for the next three weeks, I will be busy over the next three weeks.

Thanks a lot for your help in advance and Cheers!
Hyuwnook

Question

over the last three weeks Or for the last three weeks
over the last three weeks Or for the last three weeks
Which one is correct?
And is it also fine to write 'past' instead of 'last'?. For example, I have been busy over the past(last) three weeks.
What if I want to talk about future? For example, I will be busy for the next three weeks, I will be busy over the next three weeks.

Thanks a lot for your help in advance and Cheers!
Hyuwnook

Answer

You can use 'over' for both the future and the past. Over the next three weeks I will be studying for my final exam. 'For the next/last/past three weeks' is also fine. You could also use 'during' instead of 'for' or 'over'.

Here's a discussion about the nuances of the difference between 'for' and 'over': https://english.stackexchange.com/questions/165260/what-does-it-mean-to-say-you-are-doing-something-over-the-next-3-weeks

Answer

In the future 'over' does not work.

I have been busy for the past three weeks.  Last works just as well.  
You can say 'over' but for sounds better, especially academically.