k-3 not available until June

It is up to you how you want to handle this- since it is a new k-1 you may not have historical data.  You can file for the extension (but make sure you make any tax payment due by the April deadline) or file without it and amend later if needed.  In TurboTax, you should uncheck the box if you want to file it now.

is there a cover letter? it might tell you if there was foreign-sourced income. if none and you are a US citizen they'll be nothing to report when you get the k-3. so if this is your situation i would suggest unchecking the k-3 box - a must if you want to e-file. you could always amend it later if needed. it's unlikely your taxes would be higher.  if there is an amount on line 21, that is from foreign-sourced income so the k-3 will be needed. to e-file before you get the k-3 you would have to leave off the FTC and uncheck the k-3 box. yo could amend later to claim the FTC. it's your decision. 

The only way to finalize a return in TurboTax (without the K3) is to falsify the data by unchecking that Box 16 of the K1 is checked, otherwise the software will not let you proceed. This needs to be changed in the software to eliminate false reporting to the IRS. 

Also, these responses encouraging falsifying input data and amending the return later are interesting, in that Who is going to pay for filing the Amended return later? Not an Intuit concern but for a taxpayer, a wholly unacceptable combination of circumstances.