3 Tweets a day should mean 30% of followers are exposed to one of an account's tweets each week

"[...] We saw that brands that tweet two to three times per day can typically reach an audience size that’s equal to 30% of their follower base during a given week. This indicates that Tweet consistency is a key factor when it comes to maximizing your organic reach on Twitter.
When brands pair consistent tweeting with engaging content, there’s potential for even higher organic reach. In fact, Wheat Thins (@WheatThins) achieved organic reach equal to 95% of their follower base by tweeting 2x a day per week, while Trident (@TridentGum) saw organic reach that was 5x greater than their follower base by tweeting 3x per day."

Source:  Twitter blog, 11th July 2014