Using
http://vrya.net/quilt/classic.php for calculations, I get
color 1: 1.375 yards
color 2: 1.5 yards
color 3: 1.375 yards
for 20 blocks, 12" (finished) each, no sashing, no borders, no binding, and no backing. I'd get extra, but I always cut my HST's a bit bigger and trim to size. Nice block!