#include <Wire.h>

const int BUTTON_1_PIN = 2;
const int BUTTON_2_PIN = 3;
const int BUTTON_3_PIN = 4;

void setup() {
  pinMode(BUTTON_1_PIN, INPUT_PULLUP);
  pinMode(BUTTON_2_PIN, INPUT_PULLUP);
  pinMode(BUTTON_3_PIN, INPUT_PULLUP);

  Wire.begin(0x08); // Join I2C bus with address #8
  Wire.onRequest(requestEvent); // Register event
}

void loop() {
  // Nothing needed in the loop
}

void requestEvent() {
  byte buttonStates = 0;
  if (digitalRead(BUTTON_1_PIN) == LOW) bitSet(buttonStates, 0);
  if (digitalRead(BUTTON_2_PIN) == LOW) bitSet(buttonStates, 1);
  if (digitalRead(BUTTON_3_PIN) == LOW) bitSet(buttonStates, 2);

  Wire.write(buttonStates); // Send button states to master
}