My K-1 this year says a K-3 is attached but it won't be available until August. TurboTax won't allow me to file my taxes without it. How can I file my taxes before 4/15?

Delete/remove the K-3. Most forms K-2 and K-3 forms do not need to be filed this year.

The IRS announced on Tuesday February 15, 2022 that most K2/K3 reporting for 2021 can be delayed until the 2022 tax return filing.

This is an extract of the IRS announcement:

The IRS intends to provide certain additional transition relief for this year from the Schedule K-2 and K-3 reporting for certain domestic partnerships and S corporations with no foreign activities, foreign partners or shareholders, and without knowledge of partner or shareholder need for information on items of international relevance. For 2021, these qualifying domestic partnerships and S corporations will not have to file the new schedules. We are taking this step in response to feedback we received from the tax community and our stakeholders.  The IRS will provide full details of this relief soon.

Source: MinhT1

the IRS has waived k-3 reporting for 2021. the problem comes about if that k-3 reports foreign tax credits. there really isn't a way to claim those credits without the form. however, in many cases, there will be no FTC. your options are to file now and amend if there is FTC or file an extension and wait for the partnership to provide the k-3 info. To file now uncheck the k-3 box on the Turbotax k-1 form.