Arduino Mega 2560 multiple slave via SPI I'm new to Arduino and microcontroller . I want to connect my Arduino Mega to three slaves which Arduino Unos. I know that these slaves will share MISO, MOSI, and SCK lines. However, SS is unique to each slave. 

For single slave, the SS is pin 53. So, for three slaves, which other two pins can I use as SS?

Thank you for your help.
Any you like. There's nothing magic about them, as long as you have the main SS pin as an output to ensure the SPI port is in master mode.