Interfacing stm32 with MAX232
The manual (https://www.st.com/resource/en/datasheet/stm32f030f4.pdf) on page 28 has the info about the I/O structure:
  

Page 29 tells you what you need to know about your PA3 pin:
  

I am sorry to inform you that it is not 5V tolerant.

You could use a simple voltage divider on the USART input (RX) to bring it down to a more compatible level.  

Better yet, you could use UART on other pins as their alternate function. If you look at the same table 11 that starts on page 28, you will start seeing USART in the "Pin functions" > "Alternate functions" column. Match that with the "FT" or "FTf" abbreviations in the "I/O structure" column, and you've got yourself a 5V tolerant UART interface.
(Thanks to @Justme who kept on insisting that we dig for more information!)