The stimulus questions are asked in the Federal Review section of the TurboTax program. Any payment you were eligible to receive based on the 2020 Federal tax return but did not receive will be entered on the Federal tax return, Form 1040 Line 30 as a tax credit. The credit is included in the federal tax refund or it will reduce the amount of federal taxes owed.
Yes. The year you got it doesn't matter. If you don't report it Turbo Tax will give it to you again, which would be wrong and double.
If you did not get the first or second check then enter a 0 in that box. If you did get a payment (even in 2021) then enter the amount you got for each one.