My mother died In 2016 who was a foreign person. In 2017, my two siblings (foreign person) and I (US person) submitted an official agreement document for division of inheritance to National Tax Agency and paid tax to that country. However, soon after, one of sibling argued to change it. So, we put this on hold and again we are discussing how to divide assets/property now. In this case, Should I file 3520 anyway by due date of tax return for 2017 (I filed Extension) as description on the document we submitted to National Tax Agency? Or, since I don't know yet what assets and how much I will receive finally because we haven't reached an agreement with division. Can I wait until final division is made? Assets/Property haven't been transferred yet. They are still on my mother's accounts and under her name.
If I am understanding your post correctly, you did not receive any inheritance at all from this person, their accounts, or their trusts in 2017. Therefore the 3520 is not necessary or required. You will not have to bother with this until you file your tax return for the tax year you actually receive any foreign assets through this inheritance.