The three chord blues progression can be either major or minor. They are mostly 12 bars long before they return to a new verse.
This progression has been around since the end of the 18th century. It still remains pretty much the same except for chord embellishment and substitution which still follows the basic rules of the I, IV and V changes in major and minor progressions.
P.S About the Roman Numerals
When I use Roman Numerals the Upper case are for major chords like I, IV and V and lower case are for minor chords like i, iv, and v.
You will see me reference these chords like this all through the site.
This is the basic progression there are also many variations but this is the basic skeleton.
Basic Blues progression Layout
Four measures of the I or i chord followed by
Two measures of the IV or iv chord followed by
Two measures of the I or i chord again then followed by
Two measures of the V or v chord then followed by
Two measures of the I or i chord then start over.
IV or iv
IV or iv
I or i
I or i
Major and Minor Blues Progressions
The major blues progressions are made from the 3 major chords I, IV and V of the major scale.
The minor blues can come from the Natural minor scale which is the major scale starting on the 6th note. In which case the chords would be i, ii, and v. The minor chords are actually the vi, ii and iii of the major scale.
This is a variation of the above basic skeleton using a IV(major) chord in the 10th measure
Major Scale Chord Numbering
Major Progression Major Progression
Lets make a major progression in G using Dominant 7th chords.
Here are some chord diagrams
This is a variation of the above skeleton using a iv(minor) chord in the 10th measure
This is very basic dominant chord blues progression
Three Chord Minor Progression
Here are the chord diagrams for this progression