If you are SURE it is ich (small salt grain appearance) then another 3 weeks would be necessary. The treatment you have done should be ~99% effective. Occasionally the moderators recommend copper at this point.
However, if there is actual fin destruction, I would think that further hyposalinity may not be as useful as an antibiotic. You have been doing to right treatment for ich. However if you are treating the wrong disease, or if there is a second disease present, the fish may not recover.
Perhaps you can get an opinion from the moderators. They haven't been around often recently so I have been trying to help. They have more experience then I.