Need to create the dynamic dropdown for the county, state and city fields for USA.
is there any package or way to achieve this, instead of entering the values manually in the dynamic dropdown field?
You can also have a look at the file that defines the respective list, it should be possible to enter those values directly there (assuming that you transform your list into the proper format).
On the other hand, you could also integrate google maps (either by yourself or by using store addons like this: Google Address | SuiteCRM Module).