You should already know this, as a tax professional. No offense, but it's scary to read a basic question like this. Do you use IRS resources? Google:

irs llc filing

And read the IRS results in that list.

And how do three people split 50/50, anyway? Also, be aware of bad reasons to form an LLC. Perhaps that is bad guidance from that lawyer. It happens all the time. Why do they want an LLC? What has the brother got to do with this? What is the intended operation?