What are the odds against getting three heads in three successive flips of a coin?

Question

Odds against A = (1 – 3/6)/ (3/6)= 3/5 = 3::5

Could someone check my answer and correct me if I need it please

Answer 1

Prob(3heads) = (1/2)^3 = 1/8
so prob(not three heads) = 7/8

so odds against 3 heads is 7:1