I have two questions: 1) When I filed an extension this past April 2019, did TurboTax require payment then? 2) I prepaid $3000, was that factored into my tax return amt?

Did you enter the estimated payment into your tax return?

When you requested your extension you were supposed to pay your estimated tax due.  Did you enter the payment you made into your tax return?  Go to Federal>Deductions and Credits>Estimates and Other Taxes Paid>Other Income Taxes  This will show up on line 17 of your Form 1040

Your extension gave you extra time to prepare your 2018 tax return documents.  It did not give you extra time to pay.  If you did not pay your estimated 2018 tax due by April 15, 2019 then when the IRS receives your 2018 return and your payment, they will calculate your penalty/interest and bill you later.  TurboTax will not calculate the penalty or interest.