Hmmm, I think we could well avoid grewsome debates on the pros and cons of different transliteration conventions for cyrillic ;-) But "Tri Sestry" would do me... (you might want to somehow "mark" the final vowel-sound of the second word to indicate that the stress falls on it, rather than on the first one as the norms of English might suggest... but the trouble with doing so (conventionally by placing a "grave" accent on the "y") starts to modify its sound in other languages and just causes confusion in the end).
Three countertenor sisters? I can't say I'm immediately keen on that
I think that's why T-P couldn't quite believe it was the Chekhov play, and I've got my own doubts too... those are the "golden" roles Russian actresses dream of playing
The countertenor voice struggles for acceptability in Russia - there's no 600-year-old tradition of male altos in ecclesiastical choirs to legitimise it in the way it's accepted in Britain, for example. Whether right or wrong, it would be like casting The Three Sisters with Three Fairground Bearded Ladies, and you just set yourself up for a fall. The very few countertenors there are in Russia (of whom Oleg Usov is pretty-much the best currently working) are working in a vacuum - no-one is there to teach it, there's little music in print, etc.