The question mark should be replaced by:
13:15
Because a 'group of musicians' is a:
band, and so three groups of musicians are a triband, a word which is used in vexillology to mean a flag with three coloured stripes.
Now we realise that these line images must represent...
...triband flags, with their two key properties encoded:
1. The three colours are represented by the three binary triplets. The three 0/1 columns of each triplet represent the RGB colour model - column 1 represents red, column 2 is green, and column 3 is blue, with a '1' in a column indicating the presence of that colour, and a '0' its absence.
2. The ratio of the lengths of the two dimensions of the flag, height:width.
The three examples can be interpreted as...
the flags of France, Italy and Peru:
001 | 111 | 100 is blue, white, red (since 111, the presence of red, green and blue all together makes white) - the colours of the flag of France
010 | 111 | 100 is green, white, red - the colours of the flag of Italy
100 | 111 | 100 is red, white, red - the colours of the flag of Peru
So the final combination makes...
the flag of Belgium:
000 | 110 | 100 is black, yellow, red (since 000 - the absence of red, green and blue - makes black, and 110 - the presence of red and green together - makes yellow.
And the relative dimensions of the sides of the Belgian flag are 13:15, which is the answer we seek.