I have a T3 reporting Other Income (box 26) and Adjustment to ACB (box 42). I know I can enter those on the T3 page. But how / where do I enter my original Cost Basis?

You will click on the Income tab and select "Investments" then "Capital gains". 

 

As per CRA- Any amount reported in box 42, "Amount resulting in cost base adjustment", of the T3 slip represents a change in the capital balance of the mutual fund trust identified on the slip. This amount is used when calculating the ACB reported on Schedule 3 for the property in the year of disposition

 

 

https://turbotax.intuit.ca/tips/calculating-adjusted-cost-base-6285

 

https://turbotax.intuit.ca/tips/how-to-calculate-a-capital-gain-or-loss-6345

 

As per CRA: https://www.canada.ca/en/revenue-agency/services/tax/individuals/topics/about-your-tax-return/tax-return/completing-a-tax-return/personal-income/line-127-capital-gains/calculating-reporting-your-capital-gains-losses/adjusted-cost-base.html

Box 42 on a T3 can not be entered into TurboTax as a negative number.  A negative number should be allowed in the software because that is a valid amount. 

 

It appears to me that box 42 doesn't change anything in the current tax return.  It is information that is used to calculate capital gains outside of Turbo Tax? 

 

@stefanbaldwinson Please edit your T3 in EasyStep mode, it will allow you to enter a negative number in Box 42 on a T3.