RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOB | // USART3 Flow Control
                         RCC_APB2Periph_GPIOC | // USART3 Tx/Rx
                         RCC_APB2Periph_AFIO, ENABLE); // Alternate Function

  RCC_APB1PeriphClockCmd(RCC_APB1Periph_USART3, ENABLE);



  /* Enable the USART3 Pins Software Remapping */
  GPIO_PinRemapConfig(GPIO_PartialRemap_USART3, ENABLE); // TX/PC10, RX/PC11, CK/PC12, CTS/PB13, RTS/PB14

  /* Configure USART3 Tx (PC.10) as alternate function push-pull */
  GPIO_InitStructure.GPIO_Pin = GPIO_Pin_10;
  GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF_PP;
  GPIO_Init(GPIOC, &GPIO_InitStructure);

  /* Configure USART3 Rx (PC.11) as input floating */
  GPIO_InitStructure.GPIO_Pin = GPIO_Pin_11;
  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_IN_FLOATING;
  GPIO_Init(GPIOC, &GPIO_InitStructure);


  USART_InitStructure.USART_BaudRate            = 115200;
  USART_InitStructure.USART_WordLength          = USART_WordLength_8b;
  USART_InitStructure.USART_StopBits            = USART_StopBits_1;
  USART_InitStructure.USART_Parity              = USART_Parity_No ;
  USART_InitStructure.USART_HardwareFlowControl = USART_HardwareFlowControl_None;
  USART_InitStructure.USART_Mode                = USART_Mode_Rx | USART_Mode_Tx;

  USART_Init(USART3, &USART_InitStructure);

 하드웨어 컨트롤 모드로 시리얼통신하는 예제입니다.

중요한건 아래 구문입니다. 빠트리기 쉬우니 참고하세요. 

RCC_APB1PeriphClockCmd(RCC_APB1Periph_USART3, ENABLE);