stowite:
This has been suggested several times but it has been rejected by the OP because his source and destination array elements are not necessarily contiguous. I still think this is an XY problem.
Huh
I do not want to copy all bytes only the bytes that interest me from the array.
OP does not understand arrays. Or possibly memory. He got his answer
memcpy(array2+a, array1+x, 3);
(which should probably be
memcpy(array2+a, array1+x, 3 * sizeof(*array1));
)
But doesn't understand it.