Dear Abu bakar,
Third order dispersion or in general high order dispersions like 4th, 5th…etc are important to be included when you have a narrow pulse with wide spectrum. based on its effect you have to include it or you can ignore its effect as it does not have a great effect. but In general it is better always to include all higher order effects; dispersion or nonlinearities.
there is no role for the 3rd order dispersion, I think you mean it is effect on the pulse propagation. I can say in general 3rd order dispersion might make the pulse asymmetrical in frequency domain. compensation of the dispersion generally by adding an opposite effect i.e. different sign value.
Hope that clarifies your ambiguities.
thanks
Mohamed Abouseif