The 3rd season will start on September 22 and will have 25 episodes.

Casino Heroes Season 3
Updated: As we have recently reported, the 3rd season of Heroes will premiere on the 22nd and not on the 8th as previously announced.
Chris Zatta, one of the screenwriters of Heroes, has recently revealed more information about the 3rd season of the series. To start, we finally have a date: Monday September 8th. Although it is not 100% definitive, it is the date the people from the series are confirming.
The writers are already working on the new episodes, although production will not begin until May. As we mentioned earlier, the good thing about starting so soon is that they have the opportunity to shoot more episodes, and in this case, Chris has confirmed that NBC has ordered 25 episodes for the upcoming season. In principle, the first 13 will air from September to December without interruptions.
Finally, he also briefly commented on what we can expect in the 3rd season (obviously with spoilers), so click on ‘Continue Reading’ to see it.
We are going to start the season with a lot of adrenaline! Many things are happening and it’s going to be exciting. New characters, old characters you’d never expect to see again. Villains… many of them. And with all this, we will not lose sight of what keeps our Heroes connected and working to save the world.
This confirms the spoilers that said we would see old characters. Any theories? Any ideas? Any opinions? Don’t hesitate to use the comments.
Source: 9thwonders.com