When I executed the command /kill @e[type=zombie,distance=3] about 1 block away from a nearby zombie, it said entity not found.
For some reason I can't upload screenshots because I am missing a token or something.
So here are some links:
One of the photos shows me writing the command and the other one is the command not working correctly I think.
This command only checks in the radius of 3. If you want to do a radius of 3 and below, you need to replace the tag by