Pre-interview it's rounded to two decimal places. If you do the math from the GPA calculations on their website, you can see that this is true.
While they don't necessarily state that post-interview it's 3 decimal spots, intuitively it makes sense. When going down the waitlist there will be multiple students with a 3.96, 3.95 etc.. when you get to the end of the waitlist, how do you differentiate between multiple applicants who have the same GPA? You use the third decimal.
This can also be seen if you go through the waitlist invites from previous years.