Three oil stocks are falling today. None of them are terribly expensive, and one looks cheap enough to buy.Tuesday is looking like a bad day to be invested in oil stocks, as downbeat news in the oil sector takes a toll on shares of oil majors ExxonMobil (XOM -2.68%), ConocoPhillips (COP -3.35%), and BP (BP -3.30%).OilPrice.com is reporting on a "plunge" below $75 a barrel in Brent crude prices (currently$74 and change). WTI crude -- more popular in the U.S. -- is suffering a similar fall, down 3.8% at abou ...