I like to use sets of three. For a number of things, including foreshadowing.
For example, if some object is going to play into the resolution of the story, have it appear in the background, in the description or in someone's hand or whatever, twice before the point where it's going to be used for the resolution. That way, when it becomes important, readers will think, "Oh, yeah, I should have seen that coming, b/c it was there earlier." It's a variation on the old Chekov saying about "if there's a gun on the mantel in the first act, it had better be used by the third act." And it's a variation on the mystery writer's first commandment of playing fair with the reader (and not pulling the solution out of thin air, without any chance for the reader to have figured it out).
During revisions, I look for things that I can make into threes, both for foreshadowing and for running jokes and subplots and motifs patterns that carry though the entire book and tie the story together in subtle ways. And then I try to make sure there are three connected/related whatevers, sprinkled throughout the book. It doesn't have to be an object, either. It can be an idea or a setting or a situation even a series of actions that are repeated, not verbatim, but in essence, and with slightly different meaning each time, because the character/situation has changed.
JD