Dividends are not subject to Part 1 tax.  When you record the dividend income on Schedule 3, you will see a deduction from taxable income show up on line 320 or the T2 (the section where taxable income is calculated).  The Part 1 tax is then substituted with Part IV tax, depending upon the whole situation, such as dividends also paid out, etc.  If you do not record the dividends on Part 3, the program does not know that they are deductible under ITA section 112 or 113 and no deduction will show up on line 320.