You can apply a current year Capital Loss against any Capital Gains you incurred the three previous years. 

Your Net Capital Loss CarryBack amounts will not affect this year's tax return and you will not see any entries on the current year tax return.

You'll need to complete the "Net Capital Loss Carrybacks" page, and your amounts will be transmitted to CRA and they will make the necessary adjustments and reassess any years you have applied these losses towards. 

To find this section in TurboTax click the "FIND" button, search for "Losses" and complete the section that asks how much and which year you would like to "carryback" your losses to. If you have not used TurboTax to import your information from the prior year's tax returns, you may need to manually enter the amounts of Capital Gains from previous years. You can find this information on your previous Notice Of Assessments (NOA) or by viewing it in your online CRA My Account.