Issue 403 of Doctor Who Magazine has recently been released and contains some info on SJA Series 3.
"Work has been underway for quite some time, in the hope of this commision, and the desks are already piled high with scripts. Phil Ford is back at work on the series, joined once more by Gareth Roberts and Joe Lidster, with some new names to be announced in the New Year."
"There will be old monsters, new monsters, and a series finale that's already got me [Russell T Davies] excited before a single word is even written! Nikkie Smith will be producing again, with Gary Russell as script editor."
Elisabeth Sladen, Thomas Knight, Daniel Anthony and Anjli Mohindra all reprise their roles in the next season.