Part III of Form 8606 is only to be used for reporting nonqualified distributions from Roth IRAs. Absent such a distribution, Part III is to be blank.
As Opus 17 said, tracking Roth IRA basis is your responsibility. TurboTax helps you with this by maintaining the details on the IRA Information Worksheet and only uses that information when necessary to prepare Form 8606 to report nonqualified Roth IRA distributions.
