This would never work for me. Sometimes I attend talks to look for specific information, which is often a lot more than three things. Sometimes I attend just to learn about a new subject, or for some inspiration. Often, it's not a talk but a dialogue, and many interesting points come up during the Q&A. If I needed answers for a specific point, I'd ask the question.

Imposing such an artificial and rigid structure when attending a talk will stress me out, get me less, and turn me off. I still remember my student days when the best lectures or talks served to inspire, and I'd often dig deep into a subject because of that.