I wonder if your purchased feathers really weigh 1.5 grains? I didn't think anything was that light! I weighed a few of my own purchased feathers, and they weigh +- 3 grains for 4” feathers. Of course, even using my weight, you would have a difference of 3.5 grains per feather, or 10.5 grains total if you're using 3 feathers. Still, I wouldn't think that 10.5 grains more on the back end would “radically” change the dynamic spine. I would check for quill contact to see if something else is throwing off your POI.
If your shafts aren't tapered, you could consider tapering the back end which would reduce weight and maybe improve the arrow as well.