The Realbench stress test has always caused my GPUs up into the higher temp ranges, so I didn't think about it further.
I'll try it out with Furmark and update.
:::UPDATE:::
5 minutes using Geek3D Furmark - same thing is happening. Temps are just a little higher.

And after a half-hour on Skyrim running at max graphics settings.

:::ADDITIONAL UPDATE:::
I have just finished testing the system by running the Heaven Benchmark under several configurations.
Say the original card setup was 1:2:3, top:middle:bottom, and that's what I used for the aforementioned reports.
I moved cards to the following configurations, ran the benchmark, and had the same results as before:
1:3:2, 2:1:3, 3:1:2, 3:2:1 - all temperatures high:low:high, relatively speaking.
So, it must just be the way the work is distributed to cards in TriFire, and I suppose this thread can be retired now.
On a side note, my best score in Heaven was achieved with the 2:1:3 config, with all settings set at required values for the affiliated forum:

Dr. Z