You're correct that, if you had a gain on both lines 15 and 16 of Schedule D (Form 1040), you would generally enter the amount from line 3 of the worksheet in Part III. See Foreign Earned Income Tax Worksheet -- Line 11 in the Instructions for Form 8801.
But something else to consider is whether line 10 of Form 8801 -- which reads Subtract line 9 from line 4. If zero or less, enter -0- here and on line 15 and go to Part II. Form 1040-NR filers, see instructions -- has an entry.
If line 10 is zero, you wouldn't complete the Foreign Earned Income Tax Worksheet -- Line 11, and the scenario in which having a gain on both lines 15 and 16 of Schedule D wouldn't come into play.
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