According to producer Peter Bennett, the newcomers to the show are:

Alice: Played by Lucy Cohu, Alice is described as "a woman keeping many secrets from the past".

Bridget Spears: Susan Brown plays "a character vitally connected to the government, which plays an important part in this story," says Bennett, which may explain why I thought "Harriet Jones," when I first read the name, inexplicably.

Lois Habiba: The amazingly named Cush Jumbo takes on the role of "a secretary who hacks into some vital information," according to Bennett. Some vital information kept on governmental databases, perhaps...?

Doctor Rupesh Patanjali: Rik Makarem is "a junior doctor at St Helen's hospital who gets drawn into Torchwood's investigations."

So now we know that the third season has something to do with a hospital, the past, and the government... oh, and also a hacker who discovers some vital information. Put that together with the title of the season, and I'm hoping that we're about to see a season where a junior doctor at St. Helen's hospital discovers that everyone in the British government is actually an alien, only to be killed by the government, only for that to be uncovered by a secretarial hacker, who... Okay, I admit it. We don't really have enough information to make that good a guess yet.

The new season of Torchwood makes it to British screens next year.

New 'Torchwood' cast members revealed [Digital Spy]