Biodiesel, Renewable Diesel, and Alternative Fuels Extension Act of 2017 This bill amends the Internal Revenue Code to extend and modify the tax credits for biodiesel, renewable diesel, and alternative fuels. The bill extends through 2021: (1) the income tax credit for biodiesel and renewable diesel used as fuel, (2) the excise tax credit for biodiesel mixtures, (3) the excise tax credit for alternative fuels, (4) the excise tax credit for alternative fuel mixtures, and (5) the payments that are equivalent to the excise tax credits for biodiesel mixtures and alternative fuels. Beginning in 2019, the bill phases down the credits for biodiesel, biodiesel mixtures, alternative fuels, and alternative fuel mixtures by reducing the credits by specified amounts.