Selection of Working Batteries for Flashlight

What was the task given?

Divide batteries into 3 groups - 2 of them having 3 batteries each and 1 with 2 batteries. 

Then the working pair of batteries has to be in 1 of these groups and now it's easier test to each group. That is 4 working pairs might be distribute as - 
(2,1,1) or (1,2,1) or (1,1,2).

If A, B and C are name of these groups then possible combinations for testing group A are - 

A1-A2, A1-A3, A2-A3

Similarly, for B group testing pairs are - 

B1-B2, B1-B3, B2-B3

And finally, if we don't find any working pair in above testings then the C1-C2 pair of group C has to be working pair.

You may find in the working pair in testing those 6 pairs from group A or B or can conclude that C1-C2 is the working pair.