Total number of cases n(S)=23=8
Favorable cases ={(H,T,T),(T,H,T),(T,T,H)} n(F)=3 ∴ Required probability =83
Alternate Solution:
Probability of getting head in one coin is p=21 ⇒q=21 ∴ Probability of getting one head in three tosses =3C1(21)1(21)2 =3(21)3 =83