When a carb equipped car sits for 3 weeks like that the fuel bowel can dry up , all the gas evaporates. That can prevent it from starting unless you pour a little gas down the air intake of the carb, as @Yosemite suggests above.
My Ford truck sometime suffers this same malady after sitting too long, and that usually works for me. Occasionally doing that allows it to start, but it quickly dies. Add gas again, starts fine, then quickly dies again. When that happens I pop the top off the carb for a quick look-see. Usually the problem is the float or gas inlet valve is sticking in the shut position, so the fuel pump can’t deliver any gas to the carb.
There’s at least a dozen other reasons why you’d get this symptom, but the above is a good place to start.