Need form 1120 for back taxes in 2017, 18 and 19 for a professional org. It was made a 501c3 Public foundation in 2019. How do I download right version of turbo tax?

Form 1120 requires TurboTax Business.  Click here for the website to purchase the 2017 and 2018 tax year TurboTax Business products, and click here to purchase the 2019 tax year TurboTax Business product.

 

You should be certain that prior to the time your professional organization became a 501(c)3 entity it was a "for profit" corporation versus a "nonprofit corporation".  Nonprofit entities, both 501(c)3 and other forms of nonprofit entities, file Form 990.