Not everyone has all of the possible six schedules that could have been part of your 2018 tax return. If you do not have a Schedule 3 it is because you did not enter any data that required it. Your 2018 tax return will print out with all of the forms and schedules that YOU needed in order to file your 2018 tax return.
But if you want to see what Schedule 3 looks like:
https://www.irs.gov/pub/irs-pdf/f1040s3.pdf
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
