How to find the USART TTL levels of this board?
Since most of the MCU I/O is connected to the header pins (sometimes via jumpers) without a level converter in between, you need to look at the MCU datasheet.
USART3 has TTa type pins for the first MCU I found, you need to verify it for your exact MCU. TTa type pins are 3.3 V tolerant, so you want to buy the 3.3 V FTDI (which is the one you linked to).