Reaction of ethylene oxide with $\mathrm{CH}{}_3\mathrm{MgBr}$ in dry ether, followed by $\mathrm{H}{}_3\mathrm{O}{}^{+}$, gives

The nucleophilic $\mathrm{CH}{}_3{}^{-}$ opens the ring at a ring carbon, giving $\mathrm{CH}{}_3-\mathrm{CH}{}_2-\mathrm{CH}{}_2-\mathrm{OMgBr}$. Hydrolysis gives propan-1-ol, a primary alcohol containing one carbon from the methyl group plus two from the epoxide, three in all.