The BBC Press Office have now released a Description and a Trailer. They also released a promo image which you can see below
Here's the press release.
Former companion to The Doctor and investigative journalist Sarah Jane Smith returns for a third series of alien-busting adventures on CBBC this autumn.
This exciting series includes a special appearance from The Doctor, David Tennant, who joins The Sarah Jane Adventures – not just for a cameo appearance but for a full-blown starring role across two episodes
Sarah Jane's team includes her adopted son, Luke; his streetwise pal, Clyde Langer; their schoolmate, Rani, who lives opposite and has aspirations to become a journalist like Sarah Jane; and Mr Smith, their Xylok supercomputer, who resides in the attic. Add to that Sarah Jane's robot dog, K-9, and the team is set for six weeks of brand-new adventures.
Highlights include the introduction of a new alien race, the reptilian Veil, and their enemies the Judoon; an extraterrestrial girl who can make people play games against their will; a living painting; the inhabitants of a supposedly haunted house; monsters who want to infect the Earth with a strangely aggressive weed; and, finally, a chance for Sarah Jane to find personal happiness with someone who could be the perfect person to complete her family.
A brand new website will accompany the third series complete with a trailer maker, monster hunt, video diaries from the cast and web games plus exclusive behind the scenes videos.