Yes, you could go back and amend the last three years, but you could also just add the basis to this year's tax return and use the basis going forward. Because you did not include the basis, you overpaid the taxes so you cannot be held liable for underpaying the taxes. Moving forward the pro rata taxable amount will still be correct. I do not see a "use it or lose it" provision that would apply (as I said, the government was not shorted) but if I find anything, I will add to the post and tag you.
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