Question about estimated taxes: additonal income in the 3rd quarter in 2021

You could pay the 3rd quarter estimated tax only if that's when you received the additional income. However, you may be penalized for late payment unless you can show that it was only in the 3rd quarter that you received the income. There is a way to do that. 

Generally, taxpayers should make estimated tax payments in four equal amounts to avoid a penalty. However, if you receive income unevenly during the year, you may be able to vary the amounts of the payments to avoid or lower the penalty by using the annualized installment method. Use Form 2210, Underpayment of Estimated Tax by Individuals, Estates, and Trusts to see if you owe a penalty for underpaying your estimated tax.

The law allows the IRS to waive the penalty if:

You didn't make a required payment because of a casualty event, disaster, or other unusual circumstance and it would be inequitable to impose the penalty, or

You retired (after reaching age 62) or became disabled during the tax year or in the preceding tax year for which you should have made estimated payments, and the underpayment was due to reasonable cause and not willful neglect.

Refer to the Instructions for Form 2210 PDF for information on requesting a waiver of the estimated tax penalty.

If in TurboTax it appears that you would have an under or late payment penalty with reference to your quarterly estimated taxes, you will be given the opportunity to use the annualized installment method to reduce or eliminate the penalty.