From the Instructions for Form 8938, page 8: "Use the currency exchange rate on the last day of the tax year"... "even if you sold or otherwise disposed of the specified foreign financial asset before the last day of the tax year." The instructions say to use the "Treasury Reporting Rates of Exchange" webpage to get applicable rates. Form 8938 is an IRS form.
Form 3520 doesn't appear to specify what rate. It is a report to Treasury, so it appears that the Treasury Reporting Rates can be used.
These are informational forms not tax forms. The gov't is interested in making sure US persons are not able to hide assets abroad without penalty.
