Simplest answer: Go shoot them and see how far off they are compared to your sights.
Or the hard way, that might not matter for squat in real life:
It's 0.3 GRAINS per inch, not a difference of 3 grams. Over a 29" cut shaft, that will make about 0.56grams difference, or 8.7grains total. Comparatively, that's about 2% difference in weight. You're looking at shafts with +/-2.0grn over the length of the uncut shaft, good for about a 1% difference.
So the difference in your new arrows vs old ones will be about twice the variation that you might have within your shafts already. If you're committed to the new shafts, then I'd adjust my sights. If you want to mix and match them, again, try shooting them together and see how different they fly.
Running the numbers, assuming your current arrows are at 300fps, you'd be looking at 303fps with the new ones. About 1% difference in velocity. I'd be surprised if your bow delivered consistent velocity several multiples larger than that.