Mark C -- I am really not familiar with the various generations of flak vests, so I'm not sure what "Gen 3" is. This past summer, though, on CAP, we wore the old style (assuming '82 pattern) flak vest. Maybe this IS Gen 3, I really don't know. We wore it pretty much everywhere, as you say.
I can tell those who haven't worn them that they are freakin HOT. This past summer was not that hot, and the only real marching we did with them on was about an 8k web march back from the ranges. But even in 20-25 degree weather, I was dying. Don't get me wrong -- I am in good shape, so my legs were good and my cardio was fine, but it felt like my whole body was on fire. Mind you, for some reason I am one of those people who sweats a lot, and I get hot really easily.
I can't even imagine wearing one for a ruck march in summer weather. I think I would be well-done at the end, if in fact I was able to finish. Any other season would be fine, as they really don't add that much extra weight, and are actually fairly comfortable (other than the aforementioned baking effect, at least for me).
Is the new body armour any cooler?
Murph