Musical Theory Abound!!!
 #102295  by Rusty the Scoob
 
The great thing about guitars and even more so with basses is that they're geometric. Ignoring that pesky high B that you guys have, the third is always up one string and down one fret for a major, or up one string and down two frets for a minor. Or on the same string, up 5 frets for a major or up 4 frets for a minor.