At grade 3 you will be expected to be able to identify and write major scales which contain up to 4 sharps/flats in their key signature. You may continue to use the semitone (half-steps) method of calculating major scales which was introduced at grade 1. However, the Circle of Fifths is a tool which, once mastered, will allow you to calculate key signatures for all major scales – not just at grade 3 but all grades up to grade 8 theory.
MusicTheoryVideos recommends that, if you haven’t already done so, you watch the following three videos. This may sound like a lot of video viewing but, as stated above, once you have mastered the Circle of Fifths, it will allow you to calculate all major scales up to grade 8 theory.