...Specify a programmer using the -c option and try again
For Arduino, the programmer switch must be -c arduino. The full arguments for avrdude, in the Atmel Studio 7 External Tools window, are:
-c arduino -p m328p -P COM3 -b 57600 -D -V -U flash:w:"$(TargetDir)$(TargetName).hex":a
See avrdude options.
