What is the main theme of act 3 in The Tempest?

The third act of Shakespeare's The Tempest can be summarized in terms of slyness and deception, which point to the overall theme of secrecy. Sly Prospero observes without being observed, learning secrets and fooling plotters. In the process, he reveals his own forethought and cleverness.
Throughout the course of the act, Prospero learns several secrets, despite apparent secrecy. For example, he watches as Miranda and Ferdinand promise themselves to one another. Since the couple is unaware of his presence, he plans to feign ignorance, despite his excitement. Through this speech, Prospero also reveals his true opinion of Ferdinand to the audience. While it would be a bit of a stretch to call Prospero's behavior "conspiratorial," Prospero does demonstrate duplicity and slyness.
In the second scene, Ariel, another unseen observer, watches Stephano and Trinculo conspire against Prospero with Caliban. Ariel confuses the plotters, who turn on one another and call each other liars. Again, the theme of secrecy is strong.
During the third scene, Ariel intervenes once more, this time after Sebastian continues in his plot against Alonso. Prospero watches invisibly as Ariel condemns the men, reminding them of their crime against Prospero and his daughter.
Throughout the third act, deception functions as a motif, pointing to the larger theme of secrecy. Sly Prospero and Ariel collaborate to foil the plots against them, while still keeping their own secret to themselves. Notably, Prospero declares his "secret" directly to the audience. Through this act, Shakespeare shows the folly of secrecy (or at least secrecy that is less than secret). It is a highly appropriate, if humorous, point, as the audience observes every whispered secret in the play.