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.
There are so many reasons why The Unbearable Lightness of Being doesn't follow a typical plotline. To begin with, the novel features several different, interwoven, plotlines revolving around several different protagonists. Even if you identify a climactic situation for one character in one plotline, it doesn't necessarily serve as a climax for the plot as a whole. Additionally, the same events are narrated more than once from different characters' points of view. The plotline is not only non-chronological, but also non-linear. Lastly, the novel is as much a philosophical work of ideas as it is a fictional story of characters, which means we can't break it into purely plot-driven stages.