Out of gasoline, gasoline mixed with oil, and diesel, which is the most flammable and which one is the least?

A fuel's flash point is the temperature at which the fuel can be ignited by an ignition source. This link shows the flash points for various hydrocarbon fuels:
http://www.engineeringtoolbox.com/flash-point-fuels-d_937.html

Of the choices you asked about, gasoline has the lowest flash point and is the most flammable.

Your question doesn't specify which 'oil' is mixed with the gasoline, nor the % of each component. For purposes of answering, I assumed the mixture was a very high percentage of gasoline and a small blend of motor oil - such as the mixture that might be used in a two-cycle engine. In that scenario, the diesel fuel has the highest flash point and is the least flammable.

If you take an open can of diesel fuel at room temperature and throw a lit match into it, the match will go out.

Yes, I was referring to a two-cycle engine mixture of gasoline and motor oil. Thanks.