Though it's implied that the Hargreeves siblings are to blame, it's actually Harlan who causes the universe-ending paradox in The Umbrella Academy season 3. The Umbrella Academy adapts the comic book series of the same name by Gerard Way and Gabriel Bá, telling the story of the titular team of super-powered siblings as they repeatedly attempt to stave off the end of the world. The Umbrella Academy's repeated apocalypse storylines see each season bleed into the next as the Hargreeves siblings' attempts to save reality only cause further issues.SCREENRANT VIDEO OF THE DAY
Season 3 of The Umbrella Academy furthers the apocalypse stories of previous seasons by introducing a paradox that threatens to consume all reality. Traveling forward through time after saving the world in the 1960s, the Hargreeves meet the Sparrow Academy, an alternate group of super-powered siblings adopted by Reginald Hargreeves in their place. Despite the two Academies' distrust of one another, they soon learn of the singularity that is consuming reality, and work together to stop it from taking place.
Related: Why The Sparrows' Numbering System Is Different From The Umbrella Academy
It's heavily implied that the Umbrella Academy cause the paradox by appearing in a timeline in which they don't exist, but there's evidence to suggest that it's actually Harlan's fault. After saving Harlan's life in 1963, Viktor imbues him with powers, and then travels into the alternate timeline to avoid creating further paradoxes. However, in doing so, Viktor leaves Harlan behind with his powers, only to later discover that he has used them to kill the Umbrella Academy's mothers in a fit of grief. This is actually the most likely cause of the paradox, instead of the appearance of the Umbrella Academy in the Sparrow timeline, as the Hargreeves siblings are led to believe.
Harlan's Powers Are The Umbrella Academy's Biggest Season 3 Paradox
The Harlan paradox is a relatively simple one: Viktor gives him his powers, then he uses his powers to inadvertently prevent Viktor from ever being born. In doing so, Harlan's survival becomes a paradox, as without the Umbrella Academy, he would never have received the powers he used to prevent their existence. This is an obvious source of quantum unrest - more so than the Hargreeves' place in the Sparrow timeline, which shouldn't inherently cause a time paradox, as it doesn't necessarily tamper with the fabric of reality.
There's also an explanation for why the singularity doesn't appear until the Umbrella Academy appears in the Sparrow timeline. To the Hargreeves siblings, the journey was instantaneous, and it wasn't until their appearance that Harlan's actions were cemented within the timeline. Viktor's absence in Harlan's life combined with Sissy's death led to the outburst that killed the mothers, and VIktor's absence wasn't technically a fact until he'd materialized in the Sparrow timeline. While the paradox appears in conjunction with the Umbrella Academy in the Sparrow timeline, it's not actually directly their fault. Instead, it's Harlan that accidentally causes the reality-ending paradox that fuels The Umbrella Academy season 3's story.