Return 3 smallest elements in a list
In the divide and conquer approach, we break the problem into subproblems and we try to solve these subproblems.
So, you can divide your problem into two subproblems till you reach one element. Now you can combine these subproblems but in each step, you know the 3 smallest elements of these subproblems.
Fortunately, Merge Sort uses the divide and conquer idea to sort an array. Please note that in each step of combining subproblems, it is sufficient to hold only three elements of each sub-lists. So, it is not necessary to sort all elements. (Just the 3 smallest elements)
Check Merge sort here:
https://www.geeksforgeeks.org/merge-sort/