Hm, well, first, you really don't want to watch a Satellite signal re-modulated over Channel 3. It will be in Mono, and you'll lose a lot of video quality. Instead, hook up the S-Video Out from the back of your satellite decoder box into the S-Video In on your MX. And get a SCART to RCA adaptor to accept the stereo RCA cables (white and red) for audio.
In this situation the NTSC tuner is, as you note, mostly useful to pick up terrestrial signals or basic unencoded cable if you have it.