If by Cubase 3 you mean Cubase 3 SX then it is easy enough to set up two separate tracks,
one of which takes the soundcard's left input as its source and the other of which takes
the soundcard's right input as its source. If you don't know how to do this then you'd
better have a good sit down with the manual.
(If you are referring to some
ancient version of Cubase VST then I can't be sure as I haven't used any Cubases that old,
but I'd be surprised if you couldn't do the same sort of thing.)