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If you have a scale like the C major scale: C D E F G A B C. A diatonic third is essentially the 2nd note from the note you are on so for C the third above it is E, for D it is F etc. etc.
You should aim to have the entire neck covered for each key of the major scale, especially if you play music that changes harmony a lot like jazz, but in the end it is useful to master in all genres. To keep things simple I’ve chosen to use this basic C major scale position at the 8th fret because it is one that is very often used as one of the first.
When you play the above, say the chord you are playing loud. Repeat this daily (like 10 to 15min/day) and once you know this by heart, start playing chord progression you know, eg. I - IV - V or any progression you want. but still say the chord name loud while you play the corresponding triads.
Try to change also the sequence of the notes you play. eg. instead of C-E-G for the C chord play C-G-E or G-E-C or any sequence you like and do it similar for the other chords or a mix of sequences by changing it every other chord.
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