It includes 3 rounds interviews for the internship position, with the project manager, co-worker, and HR. Totally it is pretty nice and smooth, focusing on my past related experiences. However, the process is a bit stressful because it moves very fast

Start with basic screening phone interview, follow up with 2 rounds of additional interview from both you future +1 and HR. +1 will mostly ask you specific career related questions, whereas HR would ask you more about general behavioral questions.