If the software has entered a number for 3a, you can just explain that it is a software computed amount.
Please see the instructions from Form 1116, page 16 below:
https://www.irs.gov/pub/irs-pdf/i1116.pdf
Lines 3a and 3b Some deductions don't definitely relate to either your foreign source income or your U.S. source income. Enter on lines 3a and 3b any deductions (other than interest expense) that: • Aren't shown on line 2, and • Aren't definitely related to your U.S. source income.
Line 3a. Enter the following itemized deductions (from Schedule A (Form 1040)) on line 3a. • Medical expenses (line 4). • General sales taxes (line 5). • Real estate taxes for your home (line 6). If you don't itemize deductions, enter your standard deduction on line 3a.
Line 3b. Enter on line 3b any other deductions that don't definitely relate to any specific type of income (for example, the deduction for alimony paid from Schedule 1 (Form 1040), line 31a
