The third distortion coefficient (a.k.a. "K3") that Kolor has handed the control of to the user, seems not being actually enabled at least for a fisheye lens. And this is IMHO really handicapping in many and frequent cases. This lack of effect is possibly the reason why Kolor's wrote on two places on page 32 of the APG 2.6 User's manual this rather intriguing and highly disputable statement:
"3rd order : Further complex distortion. Few lenses still get this type of distortion (not very useful in practice)."!

It is quite easy to verify the lack of reaction by manually setting values such as 100000 or -100000 (instead of the present eternal stubborn NOUGHT) when Optimization setting option [3rd order] or [3rd order  + optical axis] has been "enabled" (APG 2.6 and more).
Nothing seems to ever happen on the "corrected image" when changing the value of "K3" whereas on the Panorama Editor window there is an obvious and expected effect on the same concerned image when manually changing (even slightly) the setting value of the other "K1" or "K2" correction coefficients.

Michel