Do you have to watch all three films?

http://www.vulture.com/2014/09/the-disappearance-of-eleanor-rigby-3-ve rsions-explainer.html

But my question is: which one should I see first, "Him" or "Her" ?

"the main thing is to keep the main thing the main thing"

I first watched 'Him'. 8/10
Then the next day I watched 'Her.' 8/10
Then the next day I tried to watch 'Them' but found out it is exactly the same as the first two combined, so I stopped watching.

As I have seen all three now, I can advice you to first see 'Him', then 'Her'.
This order seems the most logical, as in 'Her' stuff will be cleared up which you do not want to know beforehand.

And then do not see 'Them', as it is not needed.
The movies seperately are a way better experience to see both sides than the joined one alone, in my opinion.

Personal preference and why:

I watched "Them" first because, as anyone in a relationship knows, that the whole of a relationship varies in perspective from each partner. That's why I chose to watch "Them" first--this established the general dynamic of the relationship on a broad perspective without deep insight in to each partner, Eleanor and Connor.
From there, I say go to either Him or Her.
Personally, I chose Him because I'm wondering why the hell he is so wounded and, given he grieves differently than Her, I thought it would be a good place to start.

However, all told, I personally suggest starting with Them and YES, I think it's important to watch Him and Her, too.
Otherwise, you're only getting a portion of the story and not benefiting from the creativeness of this movie as a whole.

also, i recommend watching the three films at separate times this way the overlapping between the three doesn't seem trivial or boring.

I watched the Them one first (on Netflix) and as much as I liked this film, the actors, acting, story, and pace, I could not sit through all three of them. I don't think I could have even if I had seen them in a different order. I attempted to watch the Him and Her ones and found too many repeating scenes to hold my interest. Maybe I will let some time pass and try them again, the first one I saw really did hold my interest. The Him and Her ones don't seem to be from his and her perspectives though, just have extra scenes in them pertaining to each character.

When you get up in the morning, how do you decide what shade of black to wear? (Shallow Grave)

not really you could easily skip "Them"

Libera te tu temet ex inferis.
pro ego sum diabolus, pro ego sum nex.

you can watch either Him and Her or you can watch Them. i recommended watching Him and Her because 1. there are more details that can help you understand story better and 2. because of proper ending. Them suggest specific ending while Her actually delivered it