You should amend the 2015 return as $3,000 can be used there first or more as it can first offset capital gains for that year.
In TurboTax to enter it, you do the following:
Click
on Federal Taxes
Click
on Wages and Income
Click
on I'll choose what I work on
Scroll
down to Investments
On Capital Loss Carryforward click the start or update button
I would first amend the 2015 return to have the correct amount of the carryforward.
See below for amending:
https://ttlc.intuit.com/replies/3288565
