Hi Stuart, I know that I am answering an old question, but I wasn’t completely satisfied with all the responses you received and I thought that others might also be confused. The reason you might be confused is that it sounds like people are saying that there are 3 chords in the key of A. What they mean is that there are 3 Major chords in the key of A. If you have been following the course then you know that there are only 7 letter names for notes A to G. The key of A starts with the letter name of A. A is the first letter and assigned to the first note in the scale or note sequence. B and C do have chords assigned in the key of A, but they are minor chords. Then D and E are major chords and F is a minor. (Technically in Key of A the Cmin and Finn are sharped (raised one semi-tone in pitch) but you would need to know some music theory to understand why). G is a diminished chord and is G# in the key of A, but like Justin says, we won’t worry about the 7th chord in the scale. Also, in blues progressions, you will actually be using the A7, D7 and E7 chords.
All that is needed at first is to know the chords used in a progression. If you actually are trying to know why, then you would need to spend a year studying music theory. It is easier to understand if you learn theory for the piano first. So get a beginning piano book.Also, get Justin’s Theory Course where all of this is explained. Also if anyone is interested I put a summary in my learning log: Chords in Keys in SteveL learning log
I hope that you found your answer, but more important, that you are having fun.