I think you may be a bit confused, you are not doing 3 30 minute interviews. that just means 3 candidates will be interviewed in that timeframe one after another. As for interview questions, no one is going to be able to share specifics with you. Every school is very explicit that discussing interview questions is prohibited.
As another user said above, it is very conversational, you will be able to direct the flow of the conversation as you wish based on how you answer. I don't think there is a "right way" to answer anything. The most I can offer without violating policy about sharing questions is that my 1:1 questions were primarily centered around exploring what I described in my primary application in more detail. That being said anything is fair game and this is also likely dependent on who interviews you.
I am a rambler myself, just periodically check yourself by looping you interviewer back in. Ask them if they want you to elaborate on any specific point after talking for some time. At least that is how I try and do it.
All the faculty I interacted with were very kind and relaxing, try not to stress too hard.
