A couple of questions for the more experienced.

[LikeABoss]

So please bare with me guys as I have a couple of questions to ask, and they are very irrelevant to each other.

1) When flying an approach, I notice on the FMA it says Cat3 single, and sometimes Cat3 Dual.
We were NOT doing a cat 3 approach though.
Is this just the limit of what kind of landing you can do?

2) When landing, should we be looking at barometric altimeter or radio altimeter?
Once the QNH is set, the baro could read 2000 feet (or whatever it is), whereas the radio altimeter will always read height directly above threshold.

3) When flying a precision approach we call the minimum DH, when non precision we call it MDA.
Height is right above ground level, altitude is corrected for MSL.
Whey don't we stick to either height or altitude?

Thanks guys
Sorry for what some may find silly questions.