How to find the Phase difference?

Hi all!!!

I have a 3 phase signal that is 120 deg apart from each other. The target is to continiously monitor this signal and find if there is any deviation from 120 Degrees at any point.

I have taken 2 signals as reference (Cos and Sin waves) to find the phase difference and refered a lot of documents here which was seldom helpful. Kindly suggest me a proper algorithm/mfile to carry on with the process. Be it sampling/taking FFT i.e. Process!!

Thanks Harish.R