A 3lb flat will smoke very quickly; not much fat to render (or stall) and you'll see good smoke flavor as the penetration will have a beneficial surface to mass ratio.
The injection is very close to what I play around with, though I prefer to add some fat to it (melted or squeeze butter, olive oil, bacon grease) to help really give the internal muscle some flavor and juiciness.
250 is fine with this small and lean a cut; I'd recommend backing the temp down a bit if you go with a full packer or even full flat.
Let us know how it turns out!