The first $3,000 of net losses should have been claimed in 2010. Any additional loss could carry forward to 2011, limited to $3,000, and so on. The only way you would still be able to use this loss to offset taxes in 2017 is if the original amount was more than $21,000. If that's the case, you would probably need to file amended tax returns if it's even still possible to do that all the way back to 2010.