Pretty much, I think!
For your three years old, is she used to hearing you speak German at all? I suspect that you'll have to explain to her why you are doing so, and encourage her to try and answer German when she can.
The other things you can use to support this include reading German books, playing games in German with her and teaching her words that are relevant, cartoons and songs and DVDs in German etc. German radio on.
Does your oh speak German? I've got an Anglo friend who's married to a French man, living in France. They always did strict OPOL with their daughter, plus they made English the shared home language btw the adults - on the basis that she'd hear a lot more French outside the home - at nursery, school, grandparents etc. So she made home a bit of an English bubble for her daughter. Could you and your oh speak German?
Plus... German speaking friends? Are there any groups or associations nearby? German language playgroups? And visits to relatives in Germany, who don't speak English?
The aim, as I understand it, is to arrange things so that at least 1/3 of a child's exposure to language us in the minority tongue - whether that's speaking with you, listening to cartoons, etc. I suspect it might be a bit of an uphill battle with your oldest but everything she hears will be going in somewhere.