Right. I've been farming HGEs for the last 3 hours. I have good and bad news.
The good news is nothing has changed in terms of HGE spawn rates, or the types of materials that spawn per state; I had exactly the same experience now in 3.0 as I did in 2.4. The bad news is...yeh, you guessed it, it's the same as the good news. This is not fun or interesting gameplay, however, I have confirmed that my method to get HGEs still works, and my method to get specific materials still works (including supercaps). I have made a video which I'll probably upload tomorrow showing my technique.
Military supercapacitors can tonight be found 1800ls above the orbital plane of Sesuang A (NOTE: the orbital plane of the bodies orbiting the STAR, not the orbital plane of the binary suns).
My testing went as follows.
First I went to Sol (billions/tourism/refinery/boom), as it's ol' reliable. I went 2600ls above the star and started circling high above the oribital plane, slowly spirally closer. As I reached about 1900ls, the HGEs started to pop more often. I got lots of PLAs, PHRs and PRAs, as well as a few Proprietary Comps and CDCs. I'd say about 30 of each proto, 20 Prop Comps and mayb 12 CDCs. Feeling very happy I moved on to Kruger 60 to look for more CDCs, exactly the same thing and an hour later, I'm another 30 CDCs (and a load of Prop Comps) richer. Feeling happy I went to the nearest warring independent with decent population I could find, that was Sesuang ('A' specifically), I again went to 2000ls out and started circling. Some time later I'm 36 supercaps and about 30 MGAs heavier. My HGE rate throughout all of this was about 20% of USSes were HGEs (1 in 5) once I got into my sweet spot, which I'd like to further refine as 1700-2000ls. This distance worked perfectly for three systems (and a fourth the other day when I went for PIs), of widely varying physical size and star types and sizes. I no longer believe the layout of the system affects the sweet spot.
I am able to confirm that economy doesn't seem to matter and being a binary/trinary/more system also doesn't matter.