Open-chain ethers of molecular formula $\mathrm{C}{}_4\mathrm{H}{}_{10}\mathrm{O}$ number (structural isomers only)

Ethers R-O-R' with R + R' = 4 carbons: methoxypropane, methoxy-2-propane (2-methoxypropane) and ethoxyethane. Hence 3 (the other 4 isomers of $\mathrm{C}{}_4\mathrm{H}{}_{10}\mathrm{O}$ are alcohols).