Melting points depend on how well a substance fits into a crystal lattice, and so is much harder to predict than is boiling point which follows molecular weight (approx), shape (approx). In the case of alcohols Hydrogen-bonding is also very important in melting point. Remember that the hydrocarbon chains like to get close to each other, the -OH ends like to get close to each other, and the longer the chain, the more conformations there are, so in the end its a balancing act, propanol fits in a lattice with weaker overall molecular forces than the others and so it melts at the lower temperature.