Filter common values from three separate columns
Array formula in B15:
Copy cell B15 and paste it to cells below as far as necessary.
To enter an array formula, type the formula in a cell then press and hold CTRL + SHIFT simultaneously, now press Enter once. Release all keys.
The formula bar now shows the formula with a beginning and ending curly bracket telling you that you entered the formula successfully. Don't enter the curly brackets yourself.
Explaining formula in cell B15
Step 1 - Prevent duplicates in the list
The COUNTIF function counts values based on a condition or criteria. The first argument $B$14:B14 expands as the cell is copied to cells below. This makes the formula aware of displayed values above the current cell.
COUNTIF($B$14:B14, $B$3:$B$12)
becomes
COUNTIF("Common values in three columns", {"AA"; "BB"; "CC"; "DD"; "EE"; "FF"; "GG"; "HH"; "GG"; "JJ"})
and returns
{0;0;0;0;0;0;0;0;0;0}
Step 2 - Find position of value in array
The MATCH function returns a number representing the position of a value in a list.
MATCH(0,COUNTIF($B$14:B14,$B$3:$B$12)+(((COUNTIF($D$3:$D$11,$B$3:$B$12)>0)+(COUNTIF($F$3:$F$12,$B$3:$B$12)>0))<>2),0)
becomes
MATCH(0,C{0;0;0;0;0;0;0;0;0;0}+((({TRUE;FALSE;TRUE;FALSE;TRUE;FALSE;TRUE;FALSE;TRUE;FALSE})+({FALSE;TRUE;TRUE;FALSE;FALSE;TRUE;TRUE;TRUE;TRUE;TRUE}))<>2),0)
becomes
MATCH(0,{1;1;0;1;1;1;0;1;0;1},0)
and returns 3.
Step 3 - Return value
The INDEX function returns a value based on a row and column number.
INDEX($B$3:$B$12, MATCH(0, COUNTIF($B$14:B14, $B$3:$B$12)+IF(((COUNTIF($D$3:$D$11, $B$3:$B$12)>0)+(COUNTIF($F$3:$F$12, $B$3:$B$12)>0))=2, 0, 1), 0))
becomes
INDEX($B$3:$B$12, 3)
and returns "CC" in cell B15.