The last three secret characters are the following. You can get Matt Hall by using the Lucky Cat, and scoring a great score, which is a score of at least 100 or a score of fifty less than your top score (at least). The higher of these two will be your requirement for a great score.
Matt Hall is one of the developers of the game, and his favorite character is the Lucky Cat. Another one of the developers, Ben Weatherall, can be unlocked in the same way. All that you have to do is to get a Great Score while using his favorite character, the Dark Lord.
Finally, Andy Sum, another one of the developers, can be unlocked by getting a Great Score while using his favorite character, the Mallard. These are all seven of the secret characters at the moment, but the developers have shown that they have no intention of stopping there. There should be more coming as they continue to update the game and come up with new stuff to add.
In the most recent update, one of the things that has just been added is Disco Zoo, which is an obvious and hilarious tie-in with the game of the same name. The easter egg is that as your elephant jumps around (the same one that is in the Disco Zoo logo), you will have a crowd of other animals watching you. These animals will take the place of the trees.
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Eventually, after you score about ten points or so, the disco ball will drop down and the dance party will start, and the lights will turn down low. Everything else will stay about the same, though. The Disco Zoo is acquired the same way as everything else is; through the gacha or through an in app purchase.
Notice that now with the second update, the characters that are required to earn the developers (Mallard, Lucky Cat and Dark Lord) will appear almost immediately when you start pulling the gacha arm for characters. (NOTE: there has been another update since then; head over to page 5 to find more about the characters in the third update)