Hi,
This is because the AM335x CPU exposes many functions on 2 or more pins. Each pin has 1 to 7 different functions associated with it;
e.g. pin 31 of header P8 - or GPIO0_10 can have: LCD_DATA14, GPMC_A18_MUX1, EQEP1_INDEX_MUX0, MCASP_AXR1_MUX1, UART5_RXD_MUX1, PR1_MII_MR0_CLK, UART5_CTSN_MUX1 and GPIO0[10].
Since only one function of a pin can be in use at a time the user has to choose a pin configuration that fits his needs (commonly called “pin muxing”).
The same function is assigned to multiple ports as an attempt to avoid conflicts (e.g. if UART5_RXD was only available on pin 31, you could either use UART5 or LCD, but not both; as the UART5 function is mapped to 3 other pins as well, you can still map it to one of the other pins if you need to use LCD).
UART3 RXD and TXD are exposed via four pad pairs:
G15/G16: already used for the MMC card, so its not available
L16/L17: directly connected to LAN8710A, not usable
M17/M18: directly connected to LAN8710A, not usable
C15/C18: C18 is usable (GPIO0_7), but C15 is used for MMC connection detection
So none of the selectable pins are available, that’s why its marked as not available
You can use four ports (UART1,2,4,5) + USB serial if you don’t need RTS/CTS.
If you need RTS/CTS, you should check if it conflicts with anything else you may use.
Regards,
Norbert