ISR (USART_RX_vect) {
   char ReceivedByte;
   ReceivedByte = UDR0;
   if (ReceivedByte=='0') {PORTC |= (1<<3);};
   if (ReceivedByte=='1') {PORTC &= ~(1<<3);};
}

void USART3_IRQHandler(void) {
   uint8_t tmp;
   uint8_t rcv = 0;
   if (USART3->ISR & USART_ISR_RXNE) {
      tmp = USART3->RDR;
      rcv = 1;
   }
   if (USART3->ISR & USART_ISR_TC) {
      USART3->ICR = USART_ICR_TCCF;
   }
   if (USART3->ISR & USART_ISR_ORE) {
      USART3->ICR = USART_ICR_ORECF;
   }
   if (rcv) {
      if (tmp == '0') { GPIOA->BSRR = GPIO_BSRR_BS2; };
      if (tmp == '1') { GPIOA->BSRR = GPIO_BSRR_BR2; };
      if (tmp == '2') { GPIOA->BSRR = GPIO_BSRR_BS4; };
// И другие варианты.
   }
}