“It’s elementary, Watson”... or is it. Jeffrey Hatcher’s “Holmes and Watson” kept me on the tip of my toes in guessing the real identity of Sherlock Holmes for two hours. But the real mystery is much deeper and far more intricate than I had predicted (cue in dramatic, Dun! Dun! Dunn!). Hatcher’s play features not one, but three very different versions of Sherlock Holmes played by Ted Powers, Lawrence Stevenson and Jospeh Van Deen. For those who have read Sherlock Holmes’s so-called “come back” story, “The Adventure of the Empty House” (1903), you would know which of the three Sherlock Holmes is the “real” one, but Hatcher’s play tests and plays with your knowledge of what you think you know of the Sherlockian canon and its characters.