u24_t var24; // Some assumed 24-bit (3-byte) tytpe

//                      Shift as required  Mask-off the required portion
//                      -----------------  -----------------------------
unsigned char low    =  var24              & 0xff;
unsigned char middle = (var24 >>  8)       & 0xff;
unsigned char high   = (var24 >> 16)       & 0xff;