The three tracks were not specified, but two will come from the band's eponymous debut album, joined by one song from the upcoming "I Told You I Was Freaky." Sub Pop couldn't say if they would be released as a track pack or individually.
Team Teabag also reported that they're shooting for an Oct. 20 release, concurrent with the new album and, interestingly, that they're supposed to go out via the new Rock Band Network. The last we'd heard is that Rock Band Network was scheduled for a November release.
I've emailed a contact at Harmonix to see if they can shed any light on this, and on Rock Band Network, from their end.
Update: My contact says, without specifically confirming any Flight of the Conchords DLC: "I can clarify that the tracks in the Rock Band Network store will not be available for purchase in October, though the submission and vetting processes should be well underway by that point. The closed beta of the Rock Band Network is going really well and we're excited to open up the process in the coming weeks." For more info, visit the network's site.