Relax and be pleasant

Executives in a big consulting company generally assess your people skills: communication, interpersonal interactions, whether people like you, trust you, want to work with you, etc. So you'll do better if you relax, smile and be pleasant.

It will take some time to process a job offer, and generally people have to give 2-week notice to quit a job. So 3 weeks is too short a wait to mention during the interview. Once they give you an offer, you can negotiate a start date at that time. Good luck.