For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
The three act plot analysis is not a particularly helpful approach to Electra. In this analysis, the first act ends when the hero commits fully to his deed. If you're thinking of the hero as Electra, which pretty much everyone does, then it's hard to say what the commitment is. Getting vengeance? She's been committed to that since her father was murdered.
Act II
This one is a bit more clear-cut. Act II includes the point when the hero is farthest from resolution (or in the stickiest mess possible). This corresponds to Electra thinking that Orestes is dead – indeed this does throw a monkey wrench into her vengeance plans. However, Electra does commit to carrying out the murder on her own, which means she's not resigning to defeat in any way.
Act III
Electra and Orestes re-unite and clear up the "I thought you were dead!" confusion. They make plans to murder. They play ends when they murder the royal couple.