Portable generator won't start
As gasoline evaporates it leaves a gummy varnish in the carburetor

Gasoline sitting for three years in the float bowl of a carburetor will surely lead to some varnish.  The hydrocarbons evaporate from the fuel and oxygen acts as an oxidation catalyst changing the remaining components into other compounds leaving varnish in their wake.

Varnish will coat and clog the inside of the carburetor.  Float bowl, floats, needle and seat as well as any jets the were submerged in the gas.

Clean the inside of the carburetor

I would suggest at the very least you remove the carburetor and take the float bowl off.  Get some carb cleaner with the little red straw/hose that you can use to spray into jets and and orifices of the carburetor.  Let it sit, do it again and use a bowl to capture the runoff.  You can let the carb sit in the bowl for a bit if there are trouble spots.  Use a toothbrush.  Get a thin piece of copper wire you can stick into any jets to ensure they are not clogged.  Those are your real trouble areas.  Take your time.  Be thorough or you will have to do it again.

If you have some extra carb cleaner you can spray it around inside your fuel tank to clean that out a bit too.  Spending a little time on the gas tank will pay dividends.

Replace your fuel filter.  They are cheap.

If the engine runs with starter fluid, that means you have compression and spark.  The last piece is fuel delivery.  Focus on that and you will be victorious.  

Best of luck.