X2: 1944, Super-Mystery Comics vol. 4 #1 (Ace). Japanese spy/scientist who kept lepers in a lab in caves beneath the subway system of NYC. He used the lepers' blood to infect blood and plasma shipped to soldiers in the Pacific. His actions drew the attention of Dr. Nemesis and was killed when he touched the third rail in the subway.
X-3: 1941, Exciting Comics #16 (Standard). Diabolical spy and a master of disguises, he's the Nazi man of a thousand faces. He's captured by the Liberator.
X-Atlantis: 1941, Miracle Comics #4 (Hillman). X-Atlantis is an ancient sorcerer, master intellect, and tyrant of ancient Atlantis. At some point he died or was near death, but his intellect lived own via macines that slowly sustained his body even after it mummified and his mental powers allowed him to rule all of the undersea Atlantis. When surfacer Bullet Bob Dunn inteferes and trespasses in the Hall of Aeons where his body and machines are kept, the machines revive X-Atlantis and he vows to use his powers to enslave the world. However, he is presumably killed when Bullet Bob escapes and uses his ship's ray guns to destroy the Hall of Aeons. X-Atlantis is a bald old man in robes, befitting an ex-Mummy. All of the Atlanteans have some mental ability as they communicate telepathically, but X-Atlantis' is either far greater or amplified by his machines. He boasts also that he has occult powers though he doesn't display any ability not explained by the science fiction of the strip. As this version of Atlantis has a breathable mist like atmosphere, it is safe to assume these Atlanteans do not breathe underwater.