In [1]: import math

In [2]: def stuff(rgb):
   ...:     return math.sqrt(rgb[0] + rgb[1] + rgb[2])
   ...: 

In [3]: a = [9,9,9, 1,1,1, 3,3,3]

In [4]: i = iter(a)

In [5]: b = list(zip(i,i,i)); b.sort(key=stuff)

In [6]: b
Out[6]: [(1, 1, 1), (3, 3, 3), (9, 9, 9)]