The three chords in a basic 12 bar blues in A are A7, D7, and E7. This series of chords is also respectively known as(in roman numerals)a I7-IV7-V7 progression.
To many musicians, chords and scales are interrelated because from scales do we get chords. A chord can also be related to many different scales (even at the same time), depending on it's funtion in a chord progression. Don't worry too much about that, just know that each chord (A7,D7,& E7) in a basic twelve bar blues progression has it's own respective scale.
I suggest using the following chords as an alternative to the ones suggested in the groove sequence as we will be improvising between the fourth and ninth fret:
You can also use these versions:
It is important to be able to play these chords as you can incorporate them (or fragments of them)with your improvised melodies.