A polar solvent is one in which you can have partial charge separation within the molecules of the solvent. A good example is water. One part of the water molecule is slightly negative (the oxygen) and the other part is sligtly positive (hydrogen aoms). This happens because the oxygen atom exerts a stronger attraction on the electrons in the bond between the hydrogen and oxygen atoms in the water molecule.
So when you consider other compounds, you want to see how closely they resemble a water molecule in structure, or if there are some elements in the molecule that will attract electrons more than some other elements in the same molecule.
If you consider your three compounds, propyl alcohol has an -OH group, very similar to water (H-O-H); acetone has a C=O bond where the oxygen can attract electrons more than carbon; and propane which has only C-H bonds.
So the alcohol is most soluble, propane least.