Hello A320ECAM...
Thanks for the tip, my friend ! That's pretty much what I was taught on the 737 and what I did during my time flying that type... x3 always worked fine for me, and I gotta say that the energy management on the 737 is a little bit more tricky than on the A319/320.
Recently, though, I've read in a "Getting to Grips" that the Airbus recommendation is to do (Current ALT - Target ALT) / 3 or miles to go x 3 + the appropriate margin for speed reduction.
For your example, that would be:
360/3 = 120 + 6 ~= 126nm
For big altitude changes the result is a little bit different, but not so much for smaller changes...
Let's say I want to be at the IAF at FL050 with green dot speed (200kts) and I'm now at FL100 with 250kts:
50/3 = 16 + 5 nm =~ 21nm
Or
5*3 = 15 +5 =~ 20nm
What do you think?