[HarryT]

Originally Posted by mbovenka

I get the reference to Gaul, but you have to fudge a bit to get to three parts, I think.

I'd say it's either two (Gallia Cisalpina & G. Transalpina) or more than three: G. Transpadana, G. Cispadana, G. Narbonensis (aka 'Provincia Romana', the modern Provence) & G. Comata (pretty much everything else south to the Pyrenees and north to the Rhine)

You could split G. Comata even further into G. Belgica, G. Aquitania & G. Lugdunensis, getting up to 6

The only way to get to 3 that makes some sense is G. Cisalpina, G. Narbonensis/Provincia Romana & G. Comata.

Or am I overthinking this?

You're on the right track.

In the days when Latin was taught as a standard school subject, a book which virtually everyone would have been familiar with was Julius Caesar's "Gallic Wars" - it was THE standard text. The first line of the book is:

Gallia est omnis divisa in partes tres...

or:

The whole of Gaul is divided into three parts...

which is the reference that this detective story is making in saying that like Gaul, the window was divided into three parts. It's a reference which would be completely lost to most modern readers.