There is a way to come up with that percent age.

Het X Het will yeild on average

25% Homozygous
25% Normals
50% Hets

Since its 1:4 visual the other 3 offspring when figuring in 4's (on average will be normal) two will be hets. So
3 animals = 100% 100% / 3 = 33.33%
so each animal has a 33.33% chance of being normal or a 66.66% chance of being a het.