Although they say there are 3 interviews, it is 4. The first 3 are by phone, the final is an in-person interview. The first call you get is a 'feel out' call from a recruiter or their admin. They ask you some general questions such as 'why do you want to leave the job you are currently in' and so on. It was short and informal. From there you will move through each interview generally scheduled a week apart.
The second interview is with the recruiter. They will ask questions to feel out your work ethic and personality. Following the STAR format is essential for each example, they will be looking for that, as well for it to be explained with quantifiable numbers, if applicable. Ask for feedback (which they will give regardless I'm sure), because they will look for that feedback to be applied in the next interview.
The third (official) interview is with two area managers that were in the CP position before, and now manage a group of CP's. They will ask the same type of questions as the recruiter as well as follow up with their own related to the previous. Again, it is relatively standard, nothing scary, just follow the STAR format for answering and you will do fine. Be precise with examples and detailed on outcomes.
Once you get passed the third interview, you will come in for the fourth interview which will be a panel interview. It sounds much more intimidating than it really is. There will be 3-4 people in the room other than yourself and including your recruiter. You sit down in a conference room and introduce yourself, speak briefly and open up with a short PowerPoint presentation about yourself and why Gartner should hire you. After that, you will do a mock phone call for 10-15 minutes, you will get a short scenario and they want you to play it out. See how you are on the phone.
But do not stress! Everyone has a great attitude (from my experience) and want to make you feel comfortable, they understand it is a long interview process and do not want to bog you down with more stress so feel good and confident going in! They love confident people that are ready to sell themselves as to why Gartner should hire them!
The interview process is long and can make it stressful but my advice is to relax and have that understanding going into your first interview that the process takes some time.
Gartner is a great company that wants to train new people and build internally. They take in people with all sorts of backgrounds which makes the environment and people you work with interesting, It is nice to work around smart, motivated people so be ready to come in and work!