Run the two commands below one at a time to get Samba 3 installed and to have it run on boot.
Install Samba with Homebrew
sh < <(curl https://raw.github.com/gist/1938575/df254d9a2ac83dddb04fcc7f9b634b65708aa477/
sudo sh < <(curl -s https://raw.github.com/gist/1938575/3eef666a05bffb215e34d54f0e25599a187cc7a6/install.sh)
It should. Launchd doesn't allow daemons so this would fail more often than work. -F is the way to go.
Optionally, a single launchd could have both smbd and nmbd and a second launchd could restart both when smb.conf is modified (similar to how Apple's installation worked)