Alternate timelines (Four Doctors)

Three 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. After a reign of great peace, 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 prevented 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)

Negation
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. This time, they entered the alternate Eleventh Doctor's timeline, combating the Twelfth Doctor here instead. This time, they were victorious, preventing the alternate Doctor from creating himself, averting all three of the alternate timelines. (COMIC: Four Dimensions)