My kidney stones dissolved at day 21, but it may take some time past that to finish the job.
This was after many 1-2 week long fasts as well.
3 weeks is an impressive fast, but you may need to go a little longer to get the stones. If you don't get them, subsequent fasting of shorter duration might not get them as well.
When I was done fasting 21 days, I should have kept going. most of the stones dissolved, but a couple of years later, I had a tiny 1/8 inch stone leftover, despite dozens of 1-2 week long fasts. this stone came out the hard way.
If you do stop at 21 days I might suggest a 2 week juice fast afterwards to continue the healing.