Ill restate whats been said.....
Engines need 4 things to run. 1-compression, 2-fuel, 3-spark and 4-air
Process of elimination to see where to start looking.
1-COMPRESSION:when you pull start the mower with the spark plug in, does it have resistance? If yes, compression is there.
2- FUEL: If the fuel has been sitting for 3 years in the tank, you will undoubtedly have two problems. First and most common, theres no more octane left and it wont burn properly.The easy fix is to change the fuel for FRESH from the pump, not the 3 year old can. The second and more complex is...when fuel is left in a carburetor for extended time, it tends to thicken (varnish) and clog the jets and ports inside the carburetor preventing fuel to enter where it needs to. A thorough carburetor cleaning is in order...its not HARD but you might want to have a beer ready and invite your handy friend over for that.
3-SPARK: Sandbur suggested removing the spark plug, putting the wire back on it and holding the threaded end of the plug against the engine block while pulling the cord and watching for a blue spark at the points of the plug. Blue spark? Yes? then you're good on spark.
4- AIR...the first thing you should have done is remove the air cover to examine on the carb (spray starter fluid inside) so a clogged air filter would have been eliminated already.