Back home again now. All worked well. Thanks for the reassurance and advice. The weather was dry and often sunny, so the white bag of locks spent most of the time in the position on top of the two other bags, rather than the waterproofs that I expected.
I think being a bit more aero helped and it certainly made it easier to get over a couple of narrow bridges in Belgium, although my three bags weren't much lighter than my wife's double-pannier setup, which is the other bike in this picture:
Travelogue to come once I finish writing it and download some of my photos.