Difference amplifier
In the circuit shown, you are simply out of luck. Assuming the two inputs are at different frequencies, your expected output range is +/- 300 uV. With a minus power input of zero volts, you can never get less than 0 volts out, so the zero to -300 uV portion of your output range is simply impossible. Even the with a different op amp, the positive portion of your range is not likely to work, either. Op amps with rail-to-rail outputs do have some limits, and most don't do well when operating within a few millivolts of the rails. Since you want sub-millivolt outputs, I'm afraid you're doomed to disappointment. 

What you need to do (whether you like it or not) is to provide something like -3 volts to the minus power input.