Thanks to a possible future version of the Twelfth Doctor, alternate timelines came into being after the Tenth, Eleventh and Twelfth Doctors all touched a continuity bomb. Each then saw three possible versions of reality created by each of them making a single change in their lives. (COMIC: Four Doctors)
Time Lord Victorious
For the Tenth Doctor, a reality was created where he never saved Wilfred Mott from radiation poisoning by sacrificing himself. (TV: The End of Time) Instead, he went on to conquer the universe and rule over it as the "Time Lord Victorious", succumbing to his dark side entirely. Having "fixed a broken cosmos" during his rule, he was killed by a Raxacoricofallapatorian, the result of which prevented his regeneration into the Eleventh Doctor. The original Tenth Doctor was distraught at learning his alternate self left Wilfred to die despite the Eleventh Doctor trying to comfort him. (COMIC: Four Doctors)
All of history at once
For the Eleventh Doctor, a timeline was envisioned where, instead of him and River Song touching to end the history where all of reality happened at once because they altered a fixed point in time, (TV: The Wedding of River Song) they instead lived peacefully as a married couple. As the original Eleventh Doctor noted, this reality was dying because of such, appalled his alternate self would let the Earth suffer just so he could be happy. (COMIC: Four Doctors)
Allying with the Voord
In the Twelfth Doctor's case, his alternate self no longer travelled with Clara Oswald, having told her to "go to hell". (TV: Dark Water) Instead, the Twelfth Doctor travelled alone in his aging TARDIS. Given it seemed the safest version of history, the three Doctors agreed to enter the alternate Twelfth Doctor's timeline. Upon doing so, the Twelfth Doctor took them to meet the Voord, revealing he had allied himself with them to conquer the universe. Despite the three Doctors' efforts, the Twelfth Doctor succeeded in defeating them, sending his original self back in time, rewriting their memories, to ensure his reality would come to be. (COMIC: Four Doctors)
In a last attempt to stop the alternate Twelfth Doctor, Gabby Gonzalez was transported by a Weeping Angel to the point she met Clara Oswald. Informing her the picture she saw was a lie created by the alternate Twelfth Doctor before vanishing, Clara informed the three Doctors of such. As the picture still existed, the Twelfth Doctor realised that he would still become his alternate self and they had to stop him together. This time, they entered the alternate Eleventh Doctor's timeline and used the weapon on the crashed Dalek saucer to enter the alternate Twelfth Doctor's timeline as a surprise.
Inside the alternate Twelfth Doctor's timeline, the Twelfth Doctor posed as his alternate self, gaining access to the Voord group mind. By doing so, he temporarily stopped them from intervening as the Tenth Doctor and Eleventh Doctor disabled the Voord city's force field and linked the dimensional controls of the city to the Dalek weapon that brought them there. The alternate Twelfth Doctor discovered his younger self and attempted to destroy him only for the others to intervene. The Twelfth Doctor left his alternate self with a choice: die along with the Voord when acid flooded the city in an hour or regress the Voord along their timestream and cease to exist. After Clara apologised and reminded him of who he really was, the remorseful alternate Twelfth Doctor complied and regressed the Voord, erasing himself from existence.
In the aftermath, Clara discovered that the picture was now blank, proving that they had in fact succeeded. (COMIC: Four Doctors) In order to ensure things went as they did, the Eleventh Doctor planted the Weeping Angel in the comics package with the help of Alice Obiefune. (COMIC: The Doctor Shops for Angels)