Serious Sam 3 was on GamesCom-on. There is lots of interview below.
Co-Optimus - MTV Games - MTV Games - GamesRadar - Nvidia - JoyStiq - IGN -
Benzaie

Serious Sam 3 preview is available here.

You can watch the trailer here. There is a new weapon: the Sirian Mutilator.
The official release date is October 18, 2011. You can pre-order it from Steam. Those who pre-order now getting a 10 percent discount. Those who buy the game through Steam can get an additional 10 percent off if they own an existing Serious Sam game in their Steam account. There's also an exclusive multiplayer skin depicting Devolver Digital CFO Fork Parker.

A Serious Edition of the game has also been announced, containing:
- Serious Sam: First and Second Encounter Classic, Giftable if Already Owned
- Exclusive Making-Of Serious Sam 3: BFE Video
- Exclusive Gold Fork Parker and Gold Serious Sam Multiplayer Character
- Timed Exclusive Brett Sanderson Headless Kamikaze Multiplayer Character
- Timed Exclusive DLC: Sniper Scope for AS-24 "Devastator" Weapon
- Complete Collection of High-Res Serious Sam 3: BFE Trailers and Game Videos
- Official Sound Track Composed by Damjan Mravunac and Undercode
- Digital Artbook, High-Res Posters and Box Art

(UPDATE: It's been delayed. The new release date is november 22nd.)

Serious Sam Double D realese date is August 30th, you can watch the trailer here.

Another gameplay video here.
(Spoiler alert)

You can watch a six minutes long Serious Sam 3 gameplay here.
UPDATE: New video, this time it's almost ten minutes long here.
(Spoiler alert)

Rock, Paper, Shotgun is the first place in the world to have been given a few levels of Serious Sam 3: BFE to play with. Read more here


UPDATE: Six more screenshots from here

Splitscreen mode will be available in Serious Sam 3. Read more here.

New Serious Sam Double D pictures from here and here. You should check out the high resolution version of Serious Sam 3 cover too.

You can watch the trailer of Serious Sam Kamikaze Attack here. The trailer of Serious Sam Double D is here. Some information about Serious Sam Random Encounter here.
Ron Guyatt made three posters for Serious Sam, you can see them below.
The cover of Serious Sam 3 is available. Unfortunately, it's unofficial yet.