STM32F103C8 GPIO Ports A Through G
For the STM32, the common abbreviations are PXDD - Port X Pin DD.

So yes, for PA3, you'd enable Port A, for PB15 Port B.

There are Port A to Z depending on how many GPIOs the biggest device of the family has. Each port supports up to 16 pins. So for 160 GPIOs you need 10 ports (for example).

There are some pins which are linked to a certain port as it seems, because you might encounter port H on a device which only has 24 GPIOs available.

The alternate function mapping is in the pin description of the datasheet of the specific device you are using and cannot be found in the reference manual.

I have four documents I regularly use while developing:


Reference manual
Datasheet
Programmers manual (contains core peripheral description)
Errata (because you might encounter a hardware bug)


The order is actually the order of importance for me.



Just realized you are using a F10x device.

The STM32F1xx family is the oldest one and sadly not the most descriptive one. They have improved especially in the part of the alternate function mapping, as it is quite hard to tell what the pin is actually doing on a F1xx if you enable multiple peripherals using the same pin.

For this family you have the datasheet which tells you which pin can do what. And in the reference manual, you have the description of the AFIO_MAPR which allows you to remap certain functions to other pins.