I need to file for 2019.... 2017.... and 2015.. do i need to file each separately or can i do all at once?

You might as well seek professional help TurboTax only supports their sofware for the current tax filing year (now 2019) and three years back. So you flat out can not use TurboTax to file a 2015 or older tax return.

You must file a tax return for each tax year separately. Additionally, a prior year tax return can not be e-filed. The IRS says so. You have to print, sign and mail in each tax return for each year in a physically separate envelope to the IRS.

On top of that, if you are due a refund for 2015 or earlier, you will not get it. The IRS has a statute of limitations on refunds of three years back. However, if you owe taxes for 2015 or earlier, you still have to file a pay those taxes because the statute does not apply if you owe the IRS.

Note also the same is true for your state taxes if your state taxes personal income.