There was actually a guy on Facebook from the US who just got a Compostela last month by walking from A Coruña and documenting the additional 25k at home. I don't know if he's on this group too, but he said "Our "sellos" were a couple of bars on the US bike trail hand-writing we were there into our credentials we had obtained already and a formal letter from a Catholic shrine that we were there.". (He's from Missouri, so I assume he's talking about the Katy Trail bike path.) I asked if he did anything to document the distance. He said he had that info on an app, but they didn't ask for it at the Pilgrim's office in Santiago.
I would probably do 4 days rather than 3 (A Coruña to Bruma is doable, but whew, that's a long hilly day!). But many people do it in 3.
A Coruña - Sergude albergue
Sergude - Hospital de Bruma albergue (if full, there are hotels in nearby Meson do Vento)
Hospital de Bruma - Sigüeiro (book ahead at one of the private albergues, especially in July, which is busy!)
Sigüeiro- Santiago
Sounds like you guys will have a wonderful trip! A Coruña is a great town to explore beforehand! Buen Camino!