Easy they will get to know you and see if you are interested in the role. Ask questions concerning the role to show your interest . Also do your company research before.

First interview is standard HR, asking about your resume and experience, then discussing which positions would best suit you. Second interview is specific to the position, depending on which section of the company it's in, but very similar to the first just with a different person. Third step (in this case) is a "meet and greet" with the client, mostly seeing if personalities mesh.