You don't say what happened to the house and the other 12 acres?
What happened in 2015, that you're talking about the 2015 fair market value?
It seems to me that the Simple answer is: most of the $190,000 is taxable as a long term capital gain.
Your cost basis is the $3000 you paid plus 1/2 of what you parents paid (for the 3 acres) in 1957 plus 1/2 the fair market value (of the 3 acres) in 1983.
The 3 acre share of the $15,000 paid, for the whole, in 1957 is close to nothing. The 1983 value isn't much more. Historical values found on the internet or from the property tax assessor should be close enough for your needs. Just calling your basis $3000 wouldn't be too far off.
