Iterate through an array and group off elements in multiple of 3

Iterate through an array and group off elements in multiple of 3

I have a huge array named buffer[], its size is 1000. in every index it
has a hex value like FF. how can I group them off in multiples of 3 so
that I can get a hex color value such as FFFFFF? thank you
buffer{ff,ff,ff,ff,ff,ff,ff,ff}