The first doctor on the top is the 12th doctor, (The doctor came into the posy universe and regenerated, or changed his body, into a pony and meet derpy, this takes place in the tumbler "Ask Love struck derpy"
The second is the discord doctor whooves, (Discored 12th doctor) from the tumbler, "Ask Discord Whooves" Discord whom in this tumbler in twisted cruel villen, turned the doctor opposite to himself, Or as we call him, "Discord Whooves" A vile, cowardly, selfish, and overall a jerk.
And the last one, is Zerum, the twisted mind-washed version of the doctor being controlled by Discord. From "Ask Zerum Whooves"
As far as I know, these are not related to eatch-other story wise, they are made by the same person and are all on-going.
All have very interesting stories, but "Discord Whooves" and "Zerum Whooves" have Intense Sexual content, vile language, gayness , and sometimes rather disturbing scenes, So that's why I don't follows those tumblers.
But "Love-struck Derpy" is very light-hearted, and cute.
Actually, they are the same story-wise. Lovestruck Derpy's doctor becomes discorded whooves in the future. Zerum is sort of unclear, but its most guessed that he is an alternative that also branches from Lovestruck Derpy. And...uh sorry, But, discorded whooves does not really contain anything vile, sure it implies sex and some cursing issues (and gayness is not a bad thing to be considered a warning.) Zerum does have sexual content, but i assure you there's an option to not see that stuff. But otherwise it's a rather deep blog that contains a very broken doctor. Also, this was mentioned many times from the artist (even from discorded whooves himself) the doctor's not really evil. Yes, jerk, but not a villian.