First, congrats on getting past that first round!
Now, this completely depends on both you as a person and on you and your preparedness!
I personally had all of my interviews (for all companies) in the span of 2 weeks and much preferred it. It prevented me from getting in my head, mulling things over, etc.
If you've already done 20+ cases, feel pretty confident, and did well on the first round, 3 weeks could actually do you more harm than good.
However, if you feel shaky, felt like you squeezed through, take the extra 3 weeks!
The final option you have is to book a coaching session and get a professional review. If you do this today/tomorrow the coach can tell you whether you're ready for the final or not.
Good luck :)