OK...I'll do my best, but this is definitely the deep end of the pool:

Regarding:
=STDEV.P(IF(ISNUMBER(MATCH(ROW($A$2:$A$31),LARGE((($A$2:$A$31=$N3)+ ($A$2:$A$31=$O3)+($A$2:$A$31=$P3))*ROW($A$2:$A$31), ROW(INDIRECT("1:"&SUM(COUNTIF($A$2:$A$31, $N$3:$O$3))))),0)),$C$2:$L$31))

Basically, here's what we want to do:
If a cell in Col_A matches one of the values in my list
Then use the data from that row in my STDEV function
Otherwise, ignore the numbers in that row (set each value to FALSE)

This section creates the list of rows we're interested in:
LARGE((($A$2:$A$31=$N3)+($A$2:$A$31=$O3)+($A$2:$A$31=$P3))*ROW($A$2:$A$31),ROW(INDIRECT("1:"&SUM(COUNTIF($A$2:$A$31,$N$3:$O$3)))))

Since each cell can only contain one value, we need to test for each.
That's what this section does:
(($A$2:$A$31=$N3)+($A$2:$A$31=$O3)+($A$2:$A$31=$P3))

Let's use cell A14 for the example, so the new formula is this:
=STDEV.P(IF(ISNUMBER(MATCH(ROW($A$14),LARGE((($A$14=$N3)+($A$14=$O3)+($A$14=$P3))*ROW($A$14),ROW(INDIRECT("1:"&SUM(COUNTIF($A$14,$N$3:$O$3))))),0)),$C$14:$L$14))

=STDEV.P(IF(ISNUMBER(MATCH(ROW($A$14),LARGE((($A$14=3)+($A$14=4)+($A$14=0))*ROW($A$14),ROW(INDIRECT("1:"&SUM(COUNTIF($A$14,$N$3:$O$3))))),0)),$C$14:$L$14))

=STDEV.P(IF(ISNUMBER(MATCH(14,LARGE((($A$14=3)+($A$14=4)+($A$14=0))*14,ROW(INDIRECT("1:"&SUM(COUNTIF(3,{3,4,0}))))),0)),$C$14:$L$14))

=STDEV.P(IF(ISNUMBER(MATCH(14,LARGE((1+0+0)*14,ROW(INDIRECT("1:"&SUM(COUNTIF(3,{3,4,0}))))),0)),$C$14:$L$14))

=STDEV.P(IF(ISNUMBER(MATCH(14,LARGE(14,ROW(INDIRECT("1:"&SUM({1,0,0})))),0)),$C$14:$L$14))

=STDEV.P(IF(ISNUMBER(MATCH(14,LARGE(14,ROW(INDIRECT("1:"&1))),0)),$C$14:$L$14))

=STDEV.P(IF(ISNUMBER(MATCH(14,LARGE(14,ROW(1:1)),0)),$C$14:$L$14))

=STDEV.P(IF(ISNUMBER(MATCH(14,LARGE(14,1),0)),$C$14:$L$14))

=STDEV.P(IF(ISNUMBER(MATCH(14,14,0)),$C$14:$L$14))

=STDEV.P(IF(ISNUMBER(1),$C$14:$L$14))

=STDEV.P(IF(TRUE,$C$14:$L$14))

So the values in WOULD be used.

If the formula was applied to Row_2, the formula would resolve this way:
=STDEV.P(IF(ISNUMBER(MATCH(ROW($A$2),LARGE((($A$2=$N3)+($A$2=$O3)+($A$2=$P3))*ROW($A$2),ROW(INDIRECT("1:"&SUM(COUNTIF($A$2,$N$3:$O$3))))),0)),$C$2:$L$2))

=STDEV.P(IF(ISNUMBER(MATCH(ROW($A$2),LARGE((($A$2=3)+($A$2=4)+($A$2=0))*2,ROW(INDIRECT("1:"&SUM(COUNTIF($A$2,{3,4,0}))))),0)),$C$2:$L$2))

=STDEV.P(IF(ISNUMBER(MATCH(2,LARGE((0+0+0)*2,ROW(INDIRECT("1:"&SUM({0,0,0})))),0)),$C$2:$L$2))

=STDEV.P(IF(ISNUMBER(MATCH(2,LARGE((0,ROW(INDIRECT("1:0"))),0)),$C$2:$L$2))

This example actually falls apart here, because there is no reference like: ROW(1:0).
so we'll pretend it's ROW(1:1), ok?
=STDEV.P(IF(ISNUMBER(MATCH(2,LARGE(0,1),0)),$C$2:$L$2))

=STDEV.P(IF(ISNUMBER(MATCH(2,0,0)),$C$2:$L$2))

=STDEV.P(IF(FALSE,$C$2:$L$2))

=STDEV.P(FALSE)...and none of the values in $C$2:$L$2 would be used.


That same logic is applied in the original formula to each row in the range.