Nim, 134 126 115 108 bytes

import math,future
proc g(x:seq[seq[int]]):seq[int]=lc[(lc[x[j][i]|(j<-0..7),int].sum+4)shr 3|(i<-0..2),int]

Defines the procedure g, which requires the input passed in as a double-nested sequence and outputs as a 3-element array.

A Nim sequence is an array with @ in front, like @[1, 2, 3]. An input to this procedure could therefore be:

@[@[0,0,0],@[255,255,255],@[0,0,0],@[255,255,255],@[255,255,255],@[0,0,0],@[255,255,255],@[0,0,0]]

Nim, 134 126 115 108 78 bytes

import math,future
x=>lc[(lc[x[j][i]|(j<-0..7),int].sum+4)shr 3|(i<-0..2),int]

Defines an anonymous procedure, which requires the input passed in as a double-nested sequence and outputs as a 3-element array. The procedure can only be used as an argument to another procedure; to test, use the following wrapper:

import math,future
import strutils
proc test(x: seq[seq[int]] -> seq[int]) =
 echo x(#[ Insert your input here ]#)
test(x=>lc[(lc[x[j][i]|(j<-0..7),int].sum+4)shr 3|(i<-0..2),int])

A Nim sequence is an array with @ in front, like @[1, 2, 3]. An input to this procedure could therefore be:

@[@[0,0,0],@[255,255,255],@[0,0,0],@[255,255,255],@[255,255,255],@[0,0,0],@[255,255,255],@[0,0,0]]

Nim, 134 126 115 bytes

import math,future
proc g(x:seq[seq[int]]):seq[int]=result=lc[(lc[x[j][i]|(j<-0..7),int].sum+4)shr 3|(i<-0..2),int]

Defines the procedure g, which requires the input passed in as a double-nested sequence and outputs as a 3-element array.

A Nim sequence is an array with @ in front, like @[1, 2, 3]. An input to this procedure could therefore be:

@[@[0,0,0],@[255,255,255],@[0,0,0],@[255,255,255],@[255,255,255],@[0,0,0],@[255,255,255],@[0,0,0]]

Nim, 134 126 115 108 bytes

import math,future
proc g(x:seq[seq[int]]):seq[int]=lc[(lc[x[j][i]|(j<-0..7),int].sum+4)shr 3|(i<-0..2),int]

Defines the procedure g, which requires the input passed in as a double-nested sequence and outputs as a 3-element array.

A Nim sequence is an array with @ in front, like @[1, 2, 3]. An input to this procedure could therefore be:

@[@[0,0,0],@[255,255,255],@[0,0,0],@[255,255,255],@[255,255,255],@[0,0,0],@[255,255,255],@[0,0,0]]

Nim, 134 126 bytes

import math,future
proc g(x:seq[seq[int]]):seq[int]=result=lc[round(sum(lc[x[j][i]|(j<-0..7),int]).float/8).int|(i<-0..2),int]

Defines the procedure g, which requires the input passed in as a double-nested sequence (a sequence is an array with a @ in front, e.g. @[@[0,0,0],@[255,255,255],@[0,0,0],@[255,255,255],@[255,255,255],@[0,0,0],@[255,255,255],@[0,0,0]]) and outputs as a 3-element array.

Nim, 134 126 115 bytes

import math,future
proc g(x:seq[seq[int]]):seq[int]=result=lc[(lc[x[j][i]|(j<-0..7),int].sum+4)shr 3|(i<-0..2),int]

Defines the procedure g, which requires the input passed in as a double-nested sequence and outputs as a 3-element array.

A Nim sequence is an array with @ in front, like @[1, 2, 3]. An input to this procedure could therefore be:

@[@[0,0,0],@[255,255,255],@[0,0,0],@[255,255,255],@[255,255,255],@[0,0,0],@[255,255,255],@[0,0,0]]