The 3-D Man was a 1950's hero who came about through the unique merger of two brothers, Hal and Chuck Chandler. Chuck was a test pilot who was abducted by alien Skrulls during an important test flight. Earth was seen as a strategic location in the ongoing conflict between the alien Kree and Skrull Empires, so the Skrulls were seeking information on Earth's space program and had captured Chuck to interrogate him. Chuck resisted and escaped, accidentally causing the explosion of the Skrull spacecraft in the process. While his brother Hal watched, the radiation from the explosion seemingly disintegrated Chuck, who disappeared in a burst of light. Hal later discovered, however, that the light burst had imprinted an image of Chuck on each lens of Hal's eyeglasses. Through concentration, Hal could merge the images and cause Chuck to reappear as a three-dimensional man. Chuck become the costumed adventurer known as the 3-D Man and single-handedly subverted the Skrulls' early attempts to undermine Earthly civilization.
Hal would remain comatose whenever the 3-D Man was active but remained aware of the 3-D Man's activities through a mental link. Later, a Skrull's ray weapon altered the transformation so that Hal was the 3-D Man's dominant consciousness for some time. Both brothers' minds seemed to be present in the 3-D Man at all times, but only one of them (usually Chuck) would be in conscious control of the 3-D Man's form on any given occasion.
After a brief career as a costumed adventurer, Hal decided to retire the 3-D Man, partly because he was thinking about starting a family, and partly because he was afraid his brother's consciousness might somehow be lost during periods when Hal was the 3-D Man's dominant consciousness. Hal settled down into a career as a research scientist, got married to Chuck's former girlfriend, and raised two sons. He had only activated the 3-D Man twice in recent years, once to fight a rampaging Hulk. During this time as 3-D Man, Chuck seemed to be its dominant consciousness again.
More recently, Hal began to feel a strange impulse to travel to the Himalayan Mountains of India. Unbeknownst to him, the presence of a powerful mystical artifact was calling to him. The artifact was a pyramid-shaped shard of light, one of three such shards that were created by the universe in reaction to the presence of pure evil that was cast adrift in this dimension by the extradimensional gods known as the Trion. One of these shards was captured by the Skrulls who had abducted Chuck Chandler, and its energies had merged with Chuck in creating the 3-D Man.
Chandler also heard the call of the other shard and mounted an expedition to find it. He was betrayed, however, by Jonathan Tremont, one of the expedition's guides, who claimed the shard for himself. Hal was held captive by Tremont for many years, and Tremont used the power of his shard to create the religious organization called the Triune Understanding. Tremont had discovered the true origin of the shards and hoped to use the Understanding to end the threat of the Trion's cast-off evil, which he termed the "Triple-Evil."
Tremont also sought out alternate weapons against the Triple-Evil. He used the power of the shard that was internalized in Hal Chandler to release the powers within a new follower, Delroy Garrett Junior, creating the superhuman champion Triathlon. Triathlon unknowingly held the essence of both Hal and Chuck within himself.
Ultimately, Tremont and the hero team Avengers (which now included Triathlon among their members) fought the Triple-Evil and the aliens it had corrupted. Tremont sensed that he might overwhelm the power of the Triple-Evil, to use it for himself to become as a god, and then to reward himself with dominion over all the Earth. Instead, Triathlon found the remaining third shard and, coupling it with the power within him as personified in the 3-D Man, he wrested control of the shard used by Tremont. Using all three shards, a Triathlon/Hal Chandler/Chuck Chandler amalgamation emerged and was able to destroy the Triple-Evil.
Triathlon/3-D Man used the pyramid that once housed the Triple-Evil to return the Avengers to Earth, which had since been conquered by the time-traveling Kang. They used the power of the pyramid, channeling the energies of all those fall to its assault for millennia, to fight back and ultimately Kang was defeated.
Triathlon returned to Peggy Chandler, separating himself from Hal and Chuck and restoring both of them to physical form. Triathlon found his triple-powers had remained, although Hal and Chuck no longer possessed the power to transform into 3-D Man.
Note: In at least one timeline, a team of adventurers known as the Avengers briefly formed in the 1950's before disbanding at the request of the President, who felt the public wasn't ready to learn of them and their exploits. The 3-D Man was part of that short-lived Avengers team, and their rescue of the President from Yellow Claw helped dispel the 3-D Man's reputation as a dangerous outlaw thanks to the President's influence.