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.
Since I, Robot is a collection of short stories, we're going to try something a little different here. Here, we're going to break two different stories down to show how Asimov uses (and abuses) his plots.
"Robbie"
Act I
The first act usually ends with the protagonist committed to the plot. So, in "Robbie," when Gloria's mother gets rid of Robbie, Gloria becomes committed to finding him. Notice that Gloria doesn't start the action—her mother does. That's something that seems fairly common in these stories: the protagonists get pulled in to the story, they don't start it off.
Act II
The second act usually goes from the end of Act I (the protagonist is committed to the plot) to their lowest point—the point when the protagonist is furthest from their goal. In "Robbie," Gloria is pretty much always far from her goal of finding Robbie. But her lowest moment is probably when she goes to the Talking Robot, and not even he can give her an answer (189). In fact, this is the lowest moment for Gloria, but it's also pretty bad for her mother, which helps to remind us that Gloria's mother isn't a villain—she's just a regular person who made a bad choice. So the plot reminds us that this isn't going to be a story about good vs. evil or heroes vs. villains.
Act III
The third act is where things get fixed (or totally broken), and that's what happens when Robbie saves Gloria in the factory (217). Notice how the most important action here is Robbie lifting Gloria—which echoes the part at the beginning of the story where Robbie and Gloria are playing. How does it make us feel when Asimov ends the story very similarly to how he began it?
"Little Lost Robot"
Act I
In "Little Lost Robot," Susan Calvin is committed to the plot (find the robot) when Peter Bogert and Major-General Kallner convince her that that's the best course of action. That's not what she wants to do—she thinks the best, simplest solution would be to destroy all the robots (43) rather than let one escape. But the others convince/order her to help them. As with "Robbie," the protagonist here is drawn into the story by other people's actions and external factors.
Act II
So Calvin tests the robots twice and can't find the missing one. Is this her lowest point? Actually, her lowest point (we think) happens when she confronts Kallner and Bogert—she tells them she's willing to go public and Kallner tells her that they'll throw her in jail. Great, not only is Calvin confronted with unhelpful (and possibly dangerous) robots; the humans are also unhelpful (and possibly dangerous) to her. Notice that this act also includes Calvin's request that all the robots be destroyed (220).
Act III
And what happens in the third act? Calvin figures out how to find the robot, which satisfies Bogert and Kallner, and so everyone is happy. Wait, everyone? The modified Nestors (including Nestor-10, the lost robot) are going to be destroyed, so they're probably not too happy (325). Notice how measured this story is: every act centers on the idea or the act of destroying robots. Since we're often worried in science fiction about robots destroying us, this might give the readers the idea that we have it backwards: we don't need to worry about robots—it's the robots that have to worry about us.
So, those stories are different, and you might want to analyze the others to see if this fits, but notice certain similarities. For instance, the protagonist in Asimov stories tends to get involved because of someone else (or something else, as in the case of "Runaround"). How does that affect your reading of the stories?