I was a little confused about what AC is (not Air China, so must be Air Canada) and how I can redeem A3 miles on AC for just 20,000 Japan-SE Asia.

But I gather you mean collect the 20,000 on A3, then use a Star Alliance award.

On UA's website is provides a *A chart that shows 15OW/30RT for Japan SE Asia.

However the *A chart on Aeroplan (Air Canada) is 20,000 RT, which must be what you are referring to. Looking carefully I can see that Aeroplan defines the *A differently than UA, which can be a benefit to me.

This is [good ] news to me, as I assumed all *A awards were a standard number of miles.

So if I want to book a *A award using an AC version of the chart, do I call AC, A3, or UA (where I have most of my miles)? AC is the logical answer, but I was under the impression you call the program where you miles are (and get stuck with that program's chart in the process).