I think this should work well though I might try and flip the Home language every few years since you want them to be tri-lingual. I don't have any kids yet, but my plan is to speak the less common language where we live at home and then the other language out in the world. That seems to be the approach many friends have taken as well to good effect.

Assuming these languages are actually English, Spanish and German, I would find a local club in the minority language and make some local friends where you can speak that language and your kid can come along for the ride.