How do I enter a carryover loss from 2017 on my 2019 return?

Are you referencing capital losses or something else?  Net capital losses (the amount that total capital losses exceed total capital gains) can only be deducted up to a maximum of $3,000 in a tax year. Net capital losses exceeding the $3,000 threshold may be carried forward to future tax years until exhausted. There is no limit to the number of years there might be a capital loss carryover.

On your 2019 tax return, accumulated losses from your 2018 tax return should transfer over, not 2017.   The 2017 loss should have been transferred to your 2018 tax return and that loss, if any, transfers to the 2019 return.  

You can change the number that transferred over if you need to.  To change the number, type in capital loss carryover in the search bar and then use the Jump to capital loss carryover link to be taken to the section.  Then click Edit in the upper right corner to adjust the amounts.