Watch the Major & Minor 3rds online guitar lesson by Robbie Calvo from Double Stop Chops
Major and minor 3rd double stops are the easiest to visually see follow the harmonized major scale because the chords all adhere to having major and minor 3rds in them. The major chords I-IV-V have major 3rd intervals, and therefore double stop major 3rds. The minor and diminished chords II-III-VI-VII all have minor 3rd intervals, and therefore the double stop will be the same. Let's recap on that on the fretboard and follow the patterns along the string sets.