Well 3 weeks is a long time to prepare for a speaking exam, so just learn a little each day and you'll be fine. I didn't know that you could have longer than a week to prepare for GCSE oral! (I do GCSE French and Spanish)
You simply need to practice, practice and practice as much as you possibly can.
I think the best method to do this (for me anyway) is to write down the first letter of each word on flashcards (so each question of a different one) and see how much of your oral you know by saying it out loud using those first letters only. You could also record yourself saying it and listen to it lots of times.
Good luck!