Is there only 1 guildhall for the DB unlike the other guilds scattered all over?

Can I kill 3 people in a backyard town and still get the letter for the guildhall in DF?

1. There aren't multiple Dark Brotherhood guilds as there are multiple cults and knight orders. But just like any factions, they can and do have several guildhalls across their area of influence (in their case the whole game world).

2. The place you'll be first summoned to is usually somewhat relevant to where you've committed your entry crimes, but as they're one guild nothing will prevent you from doing the first murders/quests in some backwater, then move to wherever you want, now able to spot Brotherhood meeting places in town when there's one.