Init:

I2C_DeInit(I2C3);            // Set I2C3-Unit back to default-values

// Enable I2C-Interrupts
I2C_ITConfig(I2C3, I2C_IT_EVT, ENABLE);        // Enable I2C Event-Interrupt on I2C3
I2C_ITConfig(I2C3, I2C_IT_BUF, ENABLE);        // Enable I2C Buffer-unterrupt on I2C3
I2C_ITConfig(I2C3, I2C_IT_ERR, ENABLE);        // Enable I2C Error-Interrupt on I2C3


NVIC_InitTypeDef NVIC_InitStructure;

// Event-Interrupt
NVIC_InitStructure.NVIC_IRQChannel = I2C3_EV_IRQn;    // Select Channel (I2C3 Event)
NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 1;  // Set Priority (lower value = higher priority)
NVIC_InitStructure.NVIC_IRQChannelSubPriority = 1;    // Set Subpriority, same as above
NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;      // Enable this Interrupt

NVIC_Init(&NVIC_InitStructure);          // Load settings into registers


// Error-Interrupt
NVIC_InitStructure.NVIC_IRQChannel = I2C3_ER_IRQn;    // Select Channel (I2C3 Error)
NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 1;  // Set Priority (lower value = higher priority)
NVIC_InitStructure.NVIC_IRQChannelSubPriority = 0;    // Set Subpriority, same as above
NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;      // Enable this Interrupt

NVIC_Init(&NVIC_InitStructure);          // Load settings into registers



// Configure the I2C3-Unit

I2C_InitTypeDef I2C_initStructure;

I2C_initStructure.I2C_Mode = I2C_Mode_I2C;        // Interface = I2C
I2C_initStructure.I2C_DutyCycle = I2C_DutyCycle_2;      // I2C-duty-cycle = 2
I2C_initStructure.I2C_OwnAddress1 = I2C_address_own*2;    // Own I2C-address
I2C_initStructure.I2C_Ack = I2C_Ack_Enable;        // Enable ACK-signal (acknowledge)
I2C_initStructure.I2C_AcknowledgedAddress = I2C_AcknowledgedAddress_7bit;// 7-Bit address-width
I2C_initStructure.I2C_ClockSpeed = I2C_clock;            // Clock-speed

I2C_Init(I2C3, &I2C_initStructure);            // Load settings into I2C-Registers




I2C_StretchClockCmd(I2C3, ENABLE);    // Enable Clock-Stretch
I2C_Cmd(I2C3, ENABLE);        // Enable I2C3-Unit




Interrupthandler:

void I2C3_EV_IRQHandler(void)
{
  static uint8_t rx_buffer[4];    // Buffer for storing received data
  static uint8_t tx_buffer2[4];    // Buffer for storing data to transmit
  static uint8_t tx_counter = 0;    // Transmitting-counter
  static uint8_t rx_counter = 0;    // Receiving-Counter


  switch(I2C_GetLastEvent(I2C3))
  {

  // MAster has send slave-address for sending data
  case I2C_EVENT_SLAVE_RECEIVER_ADDRESS_MATCHED:
  {
    while((I2C3->SR1 & I2C_SR1_ADDR) == I2C_SR1_ADDR)   // Clear ADDR-Flag
    {
      I2C3->SR1;
      I2C3->SR2;
    }


    rx_counter = 0; // Reset RX-Counter
    break;
  }


  // Master has send slave-address for requesting data
  case I2C_EVENT_SLAVE_TRANSMITTER_ADDRESS_MATCHED:
  {

    while((I2C3->SR1 & I2C_SR1_ADDR) == I2C_SR1_ADDR)   // Clear ADDR-Flag
    {
      I2C3->SR1;
      I2C3->SR2;
    }

    I2C_Eval_Request();    // Evaluate the received command

    tx_buffer2[0] = (uint8_t) tx_buffer;      // Store lower byte of data-answer in array
    tx_buffer2[1] = (uint8_t) (tx_buffer >> 8);    // Store higher byte of data-answer in array

    tx_counter = 0;  // Reset TX-Counter
    break;
  }



  // Master has send data-byte
  case I2C_EVENT_SLAVE_BYTE_RECEIVED:
  {
    if (rx_counter == 0)
    {
      I2C_Command = I2C_ReceiveData(I2C3);    // First received byte = command
    }
    else
    {
      rx_buffer[rx_counter-1] = I2C_ReceiveData(I2C3);  // Store other bytes in receiving-buffer
    }

    rx_counter++;    // Increment rx-counter

    break;
  }



  // Master requests data-byte
  case I2C_EVENT_SLAVE_BYTE_TRANSMITTING:
  case I2C_EVENT_SLAVE_BYTE_TRANSMITTED:

    I2C_SendData(I2C3, tx_buffer2[tx_counter]);    // Send data from tx-buffer on position of tx-counter

    tx_counter++;                  // Increment tx-counter
    break;


  case I2C_EVENT_SLAVE_ACK_FAILURE:

    I2C3 ->SR1 &= 0x00FF;      // Clear AF-Flag
    break;


  case I2C_EVENT_SLAVE_STOP_DETECTED:
  {

    while((I2C3->SR1 & I2C_SR1_STOPF) == I2C_SR1_STOPF)   // Clear STOP-Flag
    {
      I2C3->SR1;
      I2C3->CR1 |= 0x01;
    }


     I2C_Eval_Receive(rx_buffer[0] | (rx_buffer[1]<<8));    // Evaluate the received data

     break;
  }
  }  // End of switch

}