My Letter 6475 has a different (lower) amount than my Notice 1444-C. The latter matches what actually was deposited in my checking account for EIP3. Which number should I enter into TurboTax? BTW, neither value is what I should receive since my AGI is lower in 2021 than 2020.
You should report the correct amount received. It sounds like your personal banking records agree with Letter 1444-C.
This IRS FAQ states that Letter 1444-C was issued after each payment. Letter 6475 was mailed in January 2022. That being the case, you would presume that Letter 6475 would be the accurate document.
Do you have access to your online account to see which, if either, number is reported there?
IRS FAQ states:
People can locate this information on Letter 1444-C, which they received from the IRS during 2021 after each payment, as well as Letter 6475, which the IRS will mail to them beginning in late January 2022. Individuals can also view this information in their online account in January.
Turns out that amount is exactly half of what we received. Also the letter was addressed to me, not me and my wife. I assume her letter for the same amount will arrive shortly.
Yes, IRS letter-Notice 1444-C is sent separately to each person.
Please see this TurboTax FAQ, for more information, How does the third stimulus payment affect my 2021 taxes?