- Naming Substances

I'm a beginner in Chemistry and only recently have learnt 'Di' oxide means two oxygen and so forth. Anyway my problem is well basically sulfate, sulfite (as in I'm not entirely sure when you get a sulfate and when you get a sulfite) but my main problem is naming substances (mainly with brackets). For example Mg3(PO4)2, see for this I would say.... Trimagnesium Diphosphate. Can anyone tell me if this is correct. Another example is Al(OH)3, I'd say it's..... Aluminium Trihydroxide? Doesn't sound correct to me... I wish I understood this stuff. Can anyone help me? thanks