The short answer is “maybe”. Dead Space 3 is not in development right now, but Dead Space 2 is already outselling Dead Space 1. If the game will be a (financial) success, the developers already have plans for a Dead Space 3.
A lot of effort went into marketing Dead Space 2 and shortly before Dead Space 2 was available for preorder, Dead Space 1 was available for only 5 bucks on Steam to draw some more attention to the franchise. When people are interested in the game and keep buying it, a 3rd part becomes inevitable.
Dead Space 2 exec producer Steve Papoutsis already developed a plan for Dead Space 3 and the story they want to tell in the sequel, but other than that you’ll have to wait until some official sales numbers are out.
In an interview he also states that he’s doing his best to make Dead Space 3,4,8,10,20 become a reality.
”We’re working really hard to make sure this game’s better so we get a chance to do Dead Space 3, 4, 5, 6, 7, 10 and 20, but when there isn’t an outpouring of support it doesn’t make a lot of sense to do that.”
Games like Dead Space are simply too expensive to fail. That’s the advantage indie-developers have, they don’t have a million-dollar budget and therefore don’t have any financial risks.
So, if you like the game after playing the demo, go and buy it on Steam or some other place.