Short answer:
No, you can't brute force it.
Long answer:
Suppose the 3DS key is sixteen bytes long, just like the DSi. That would be 256^16 possible combinations, since each of the sixteen bytes can have 256 possible combinations. Multiply that with a calculator, and that's 340,282,366,920,938,463,463,374,607,431,768,211,456 combinations. Now for sake of arguement, let us suppose that every person in the world each has a quarter of a million dollars to spend on
one of these devices, which can test 90,000,000,000 keys per second. Since the world's population is 6,840,507,003 according to Google, then that means we can test 615,645,630,270,000,000,000 keys per second. That would take us 552,724,408,636,999,296 seconds to test every key. Divide that by 60 seconds, and we need 9,212,073,477,283,321 minutes. Divide again by 60, and that would take 153,534,557,954,722 hours. Divide by 24 hours, and we have 6,397,273,248,113 days. Divide that by 365 days, and we have
17,526,776,022 years. The universe is scheduled to last another 4 billion years, since it is currently half way through it's lifespan. So in summary,
we would need more than 4 times the time left in our universe.
Therefore, we come to the conclusion that its NOT possible to brute-force the key...