Configuring board for USART3 serial port


I have recently started working on an STM32F4_Discovery px4 firmware and I seem to be completely stumped with what i assume is an easy procedure. If I want to use USART3 pins for serial communication. From what I understand, I can’t just go off making modules for such a task from the get go - first I need to initiate the USART3 pins into alternating function mode. While I found extensive documentation on how to do this for a pure STM32F4, I can’t find anything similar when it comes to px4 firmware.

Would it be possible to get some guidelines for this?