TurboTax should have asked you about your first two stimulus checks (including the one that went out around Jan. 1) just before finishing your return if you've made it there. If you didn't enter them in TurboTax & your refund was overstated by the amounts of those checks (i.e., $1200 EIP1 + $600 EIP2 for most single taxpayers), the IRS will likely correct it as a simple clerical error without your having to amend your return.
NONE of the three stimulus checks (President Biden just signed the third one into law today) are taxable. However, you're supposed to enter the first two into TurboTax for 2020 so it can properly compute the Recovery Rebate Credit (RRC), or the backup plan for the first two stimulus checks. (The RRC will return on 2021 returns next year as the backup plan for the third stimulus check at a minimum.)
