It depends on how much of the HX711 "bandwidth" you are using. If zero to full load on your cells is using half of the ADC's capability, and if you are really getting 16 noise-free bits out of the ADC, then your noise-free resolution would be about 6 grams, not 3 grams.
You can use this formula to estimate the noise-free resolution: (full scale load) / ((2^bits)0.128S) where bits = noise-free bits and S=load cell sensitivity in mV/V, provided that the gain is 128 (the usual for HX711 channel A), S is less than 3.9 (so 0.128*S<0.5), and no load on the scale produces a differential voltage (across A+ and A-) of approximately zero (that is, you're using half of the ADC's full range).
If those conditions are true, then in your case with full scale load = 200,000 gr, bits = 16, and S=3.9 mV/V, then the noise-free resolution would be 6 gr, or 1/32,000 of full scale. For another example, if in fact bits=14 and if S=1mV/V, then NFR=95 gr, or 1/2,000 of full scale.
Also, as I mentioned in my first post, there are lots of other sources of error to consider. Achieving an accuracy of 1/20,000 or better is, according to this document, very difficult. To learn more, read that doc and Google ADC errors.