The income you paid tax on in 2018 (1/3 of the $100,000) would increase your tax basis, so that would reduce your taxable gain in 2019. So, you would pay tax on the $20,000 in 2019.
The liquidation of a partnership interest would be considered capital gain income, and as such it would be used to calculate net investment tax on form 8960. However, it would not be considered a sale of business assets, so form 4797 would not be involved.
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