Hi, Set your motor tuning to these settings both X and Y 320 1500 400 Z 400 1500 400. You should also go through all the settings on this page;
https://the-cnc.blogspot.co.uk/p/my-mach3-settings.html
These are generally the correct set-up for most 3020, 3040 and 6040. My machine is a 6040 and it took me ages to get it set-up so i started a blog with all the info so that people can just go through and copy the settings. Once the settings are done everything should work ok. If not, then it's probably not the settings and you may need to look at other things, but as I've said this is a good place to start from.
Kev