Yup, it is just half an hour before, very commonly used here. There’s some other English language (Australian?) where it means the opposite - totally not confusing.

We also use quarter to/quarter past as well of course

Yep, though we also have “Klokken halv 4” which is especially confusing for foreigners

In Denmark it’s always written in 24hr, but I’d say it’s 50/50 whether we say 3 or 15 for 15:00.

I guess saying 3 is more casual. But we never use “hundred”. 15:30 would just be fifteen-thirty.