let millie's age be "X"
then lucy's age would be "3X"
then kevin's age is 2 x (3X) = "6X"
if the sum of their ages is 120 that means
X + 3X + 6X = 120
10X = 120
so X = 120/10 = 12
by substituting, millie is 12 years old
lucy is 3x12 = 36 years old
and kevin is 6x12 = 72 years old