osx Panther 10.3.3 or higher
https://www.steinberg.net/en/support/unsupported_products/cubase_sx_3.html
ftp://ftp.steinberg.net/Download/Cubase_SX_3/3.1.0.933/Cubase_SL_SX_3.1.0.933_Version_History.pdf
the last update for sx3 was August 2005, Leopard was not released untill 2007.
i dont think it was ever updated to work on leopard or snow leopard so you would need to run it on a powerpc mac
running panther or tiger - so you would need to run it on a powermac G5 or powermac G4