Hi. I don't have the Form K-3 yet. Can I file taxes now and amend later or should I apply for an extension and file when Form K-3 is available ?

your choice but I would recommend an extension since this will avoid the need to file two returns. some find amending can be a pain.  please note that for some who will be receiving a K-3 there will be nothing to report. note that any balance due if you do extend is due by 4/18 (unless you're eligible for CERTAIN  disaster relief. that would also give you extra time to file without the need for an extension) 

You should file an extension and wait until you receive the K-3 to complete your return.

From @AliciaP1: I do not advise you to file the return without the K-3 information unless you are certain it will be blank.  The Foreign Taxes paid or accrued box being empty does not guarantee the K-3 will be blank.

If you have not filed the return and do not have key indicators to file your return without the K-3 I recommend filing for an extension and completing your return when you receive it. You cannot file without the K-3 information. The IRS regards filing without the expected K-3 as an incorrect filing.

See "Domestic Filing Exception" link under What's New section for Partnership K-1 or S-Corp K-1 depending on the type of K-1 you received.

@bogdan-dumitresc