2002 honda shadow ace deluxe, sat for 3weeks now wont start. changed battery and...
 

...it does get a spark.? I checked spark it looks a little weak to me but not sure if it supposed to l;ook like that.

Only three weeks and it won't start? Where was it sitting? Under water?No, you should get a nice fat blue spark.

It takes three things to start a vehicle ignition,fuel,and compression.I would pull the plugs to see if they look like there getting fuel,they should have a little bit of fuel on them when your trying to start it,if there soaked with fuel ,that's not good.Some times Ive taken out the plugs and squirted 30 weight oil into the cylinder turned over the engine a few times and then installed plugs.this will increase compression and will help start.Good luck to you.