BARCELONA — China's Huawei isn't the only smartphone maker adding a third display to its devices. At the Mobile World Congress (MWC) trade show in Barcelona, a number of firms were showing off their display technology innovations. The South Korean tech giant Samsung revealed its new "trifold" concept devices at the event: the Flex G and Flex S. The Flex G has three screens and folds flat inwards and outwards, a bit like a book. The Flex S, on the other hand, has a more zigzag-like shape. It's meant to resemble an "S" — hence the name. The Flex S…