The Sherlock Curse is inaccurate -- I'm fairly certain this will be the last series.
Given the stories I think they're going with and the ever-increasing popularity of the stars, they won't continue after this third installment. Benedict and Martin are far too busy now and, if they use the stories I think they're going to, then everything should wrap itself up. Well, as much as Sherlock Holmes can ever be expected to wrap up anyhow. (Besides, Gatiss and Moffat are Holmesians; they know when to let things bow out into the night and not ruin the original context by overstaying their welcome with this particular series. It's a project that's too close to their hearts to spoil it simply because it's popular, which I've always appreciated.)