Assuming that there are no special rules relating to WC cases, it is only rarely possible to reopen a case three years after it was concluded. You would have to show a huge miscarriage of justice, fraud on the court of a substantial and material nature, or something similar.
In Montana, it would be possible depending on the year of your injury. If you have not claimed any medical treatment and returned to working during those three years, it is unlikely you can prove what you need to reopen a stale claim file. You should consult with a workers compensation attorney with experience immediately to either pursue a reopening or figure out a different solution.
If you did not make a full and final settlement, in California you have 5 years from the date of the injury to reopen for new and further disability. Since you do not tell us why you wish to reopen the matter, I can not say anything more.
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