This play doesn't lend itself well to the three-act structure since Winnie begins the play at the point of no return: she is completely stuck. Throughout the play her actions never get her anywhere, and even during those brief moments when we think she might just do something, she's always interrupted, either by herself, the external world, or Willie, her husband.
It doesn't take long for us to realize that Winnie isn't going to dig herself out of her hole, yet we still have hope that maybe, just maybe, she'll actually do something. It isn't until the second act that we realize poor Winnie is as far from resolving the problem as we are from getting over the genius that is Breaking Bad.
Silence? Death? Or, as the Beckettian critic Francis Doherty proposed, the third act could be one in which the mouth is buried but the mind continues to function. In other words, if a tree falls in a forest and no one is around to hear it, does it make a sound?