The Three Sisters

Why are essays that refer to Three Sisters as The Three Sisters posted here? Is that not a sign of a lack of research?

It is not any sort of nefarious intent nor is it a lack of research.  What you see as a problem here is simply a choice of the translators from Russian to English.  Some translators prefer to use the article "the" while others stay truer to the original language by omitting the article.  I do not speak Russian, so I will leave it to another to further explain the choices made as a translator makes his or her decisions. 

I am currently rehearsing to play Andrey in Three Sisters.

And the word of my Director is that the play is 'Three Sisters' as opposed the 'The Three Sisters' bacause although this family is used to tell this story, it is not about these specific sisters but an example of what my many families were going through at the time. Its a slice of the culture.

In reply to #1:

 The Russian language contains no articles. So it's up to the translator to determine the context and choose the appropriate article.