I have just bought the The Fuller Memorandum and realized that it is the 3rd book in a series.
Is it OK to read just this one, or should I read the other 2 first?
While the series share the same protagonist (Bob Howard), they're 3 different books, even different writing styles and can all be read apart.
Where The Atrocity Archives was written in the idiom of Len Deighton and The Jennifer Morgue was a pastiche of Ian Fleming's James Bond novels, The Fuller Memorandum is a homage of sorts to Anthony Price's Dr David Audley/Colonel Jack Butler series of spy thrillers, and features two minor characters named Roskill and Panin, names which appeared as recurring characters in Price's series.
I'm just reading the forth book "The Apocalypse Codex" and I also recognize that Stross always gives short introductions into relevant aspects of the story. So, you'd still understand most of what happens without reading the other books.
But I strongly advise you to read them in order. Especially "The Apocalypse Codex" has many references to previous topics, e.g., "destiny entanglement" and such. Personally, I wouldn't have had that much fun reading it if I had read it out of order.
Read them in order for maximum enjoyment and understanding:
Also read Mr Stross' blog for further fascinating information