Configuring ADC in STM32F3
You haven't enabled the ADC clock.  Try
RCC_AHBPeriphClockCmd(RCC_AHBPeriph_ADC12, ENABLE);

If that doesn't work, use your IDE to find the declaration of RCC_AHBPeriphClockCmd and read the source to see if I've gotten the name of the clock wrong.
The STM30xx library docs  will probably be of help.  Enabling the ADC clock is step 2 in Section 3.2.1 -- and I can't see anything else you missed.  From that doc:

3.2.1 How to use this driver

select the ADC clock using the function RCC_ADCCLKConfig()

Enable the ADC interface clock using RCC_AHBPeriphClockCmd();

ADC pins configuration  Enable the clock for the ADC GPIOs using the following function: RCC_AHBPeriphClockCmd(RCC_AHBPeriph_GPIOx,
ENABLE);  Configure these ADC pins in analog mode using GPIO_Init();

Configure the ADC conversion resolution, data alignment, external trigger and edge, sequencer lenght and Enable/Disable the continuous
mode using the ADC_Init() function.

Activate the ADC peripheral using ADC_Cmd() function.
FWIW, when I have trouble, I usually go into the library declarations, as these functions tend to be very well self-documented.



FWIW, my SOP for library problems is to find the declaration in the library, as the libraries tend to be fairly well self-documented.