Sounds like something in the fuel sys. a bad coil or a timing problem would show up thru out the gear range.When you shift into 3rd thats when the engine needs the most power and uses the most gas.Many times when there is a bad fuel filter it shows up first at speed say above 40 mph.Start by checking the filter if there is one than the carb,take it apart and clean it,fuel lines,go thru all of it,one thing at a time.
Pete
