Plot hole: After the car is destroyed, Gru is knocked unconscious and Dru enters Balthazar Bratt's robot to destroy the robot's core from the inside. Once Gru wakes up, Balthazar attempts to kill him with the robot's laser and Dru destroys the core causing the robot to collapse. Gru then yells out "Dru!" and runs over to the wreckage of the robot to try and find him. The problem with this is that since Gru was knocked unconscious before Dru entered the robot, and Gru knew nothing of Dru's plan to disable the robot's core, there is no way for him to know that Dru was inside the robot.
Plot hole: After Balthazar Brat watches the commercial for his action figure that disintegrated a school book he begins having a discussion with his robot. In that discussion he says "it's too bad Grue will not be around to stop me, oh wait no it's not". He would not have known anything about Grue or Lucy getting fired, so as far as he knows they would still be around.
Plot hole: After Gru and Dru knock Bratt's robot down to the street Bratt resumes using the laser to cut the city up but the robot wouldn't be able to even see over the buildings around it, so how can Bratt even know where to fire the laser at that point, when he can't see what he's hitting?