On analyzing the given matrix, we find two patterns in each cell of a Row, Numerical and alphabetical.

• Numerical pattern: Here we saw that when we subtract column 1 from column 2 we get column 3

    Column 2 - Column 1 = Column 3 ;

                                              Row 1 : 10 (column 2) - 7 (column1)  = 3 (column 3)

                                              Row 2 : 9(column 2)  - 3 (column1)   = 3 (column 3)

                                              Similarly, same pattern is following in Row 3 .

                                              So column 3 numeriacal value for Row 3 =  13 (column 2)  - 10 (column 1)   = 3 (column 3).

                                              Hence, the Numerical part for Row 3 column 3 is '3' .

• Alphabetical Pattern:  A, B, and C are used in a cyclic manner.          

                                                So column 3 alphabetical value for Row 3 = 'B'.

Combining both patterns, we get the missing entry, i.e. ' 3B '

Hence, Missing Entry is '3B'.