Converting a recursive algorithm to a runtime function
The key is to use right - left as $n$ for the parameter of the runtime recursion. What is the size of the subparts?

Closer hint:


   Note that the algorithm separates the input part of the array in three approximately equal-sized parts; the first two are sorted recursively, the third with Mergesort. This should be reflected in your recurrence.


Almost finished:


   Assuming Merge3 runs in time $\Theta(n)$ und MergeSort in time $\Theta(n \log n)$, you get a recurrence of the form

 $\qquad\displaystyle T(n) \approx 2T(n/3) + \Theta(n) + \Theta(n/3 \log(n/3))$

 since all parts have size $\approx p \approx n/3$. Solve this with the Master theorem and flesh out the details.