Looking at this chart:
. . . it is not hard to see the Major Thirds (remembering always that a Major Third is made of two whole steps).
DO to MI
. . . is a M3
FA to LA
. . . is a M3
SOL to TI
. . . is a M3
LA to DO would
NOT
be a M3
RE to FA would
NOT
be a M3
MI to SOL would
NOT
be a M3
