Hey Steve, let us know what exactly you need from 3.x to see if what you want to do is achievable, but since you’re doing the builds in a separate system, I assume it shouldn’t be an issue to just link the images in the docker-compose that you’ll push to openBalena.
Regarding A/B flashing, I assume you mean doing hostOS updates that support rollbacks? Currently that is not supported in openBalena (which is one of the reasons we still keep it in beta), at least without some manual on-device intervention using SSH, but it is definitely on the roadmap. You can find more info how to do it here: Roadmap for BalenaOS Updates in OpenBalena, but I’ll double-check with the team if that is still up to date.