Arduino for STM32

You can optimize several things as explained in the wiki.
About code, I talk about the variant and the update you made else can't help on the clock issue. Some hardware info can help also. Like HSE? LSE?....

Hard to help without the code you add.

A Pull Request on GitHub.

You can find here how to declate the generic:
https://github.com/stm32duino/Arduino_C ... 28board%29

Feel free to submit a PR

Not necessarily as it works fine on nucleo board with same mcu.

Are you sure you used USART3? (Which pins?)
You could read the Errata Sheet, they are some point around UART, ex: Data corruption due to noisy receive line.

Hi which library you used ?
Issue is that the type used in the library is not correct to call portOutputRegister

OK. so probably issue comes before.
Hard to help more, will see if I can have this board to test on my side but not sure to have one.

Try with it enabled else you don't have the symbol for debugging.

did you build with Optimize for debug selected in the menu ?