Thanks in advance!
Hardware and performance
3000 concurrent users in a course
I have had over 5000 users in a course a number of times and everything was just fine.
3000 users doing what?
3000 concurrent users is a crazy amount. More than enough for me to ask you to justify how you arrived at that figure.
If it involves spending money, getting that wrong would be a very bad thing.
Moodle scales very well by throwing hardware at the problem. Quizzes are about as bad as it gets for system load.
That's why I asked you the question you didn't answer
If you don't know what you will be doing and what the implications are then I strongly recommend you run a pilot and get some experience (and measurements).