The third part of the series is currently in development.
While Starcraft 2: Heart of the Swarm will be out soon for PC, marking the second part in the overall saga, Blizzard is already hard at work on the third part, Legacy of the Void. During a Reddit AMA, game director Dustin Browser confirmed that work was already being done on the missions, plot and scripts for the Protoss campaign.
While Wings of Liberty focused on the human side of the campaign, culminating in Sarah Kerrigan reverting to her normal human form, and Heart of the Swarm sees her doing her part to reunite with the the Zerg, Legacy of the Void focuses on the mysterious and ambiguous Protoss.
Considering their enmity towards the humans and Zerg, it would certainly be interesting to see the universe from their perspective for once.
Heart of the Swarm releases in March 12th for PC, with Blizzard stating it “is doing everything we can to make sure this launch goes as smoothly as possible.”