As you have multiple buttons, and need to determine if the buttons are being pressed, or are pressed (the Arduino is a lot faster than your fingers!)
As wvmarle said, button state detection and debouncing is mandatory - followed by ‘scoring’ your buttons so you can determine when a valid combination has been pressed - (I’d suggest around 200mS to avoid mistriggering), only then you can choose which function to be called.
(Same when buttons are released because your fingers will come off at slightly different speeds)
If you plan it right, the buttons can accumulate as powers of two - so then you simply need to switch-case for each possible ^2 value. (0 is no buttons)