There is no set date for an official beta right now, some things still need to be ironed out. You can see a list of issues related to 3.0 here.
Additionally, while these are not official builds, you can download daily builds of the latest Godot 3.0 developments here: https://bintray.com/calinou/godot/editor/_latestVersion#files
(This is a permalink that always points to the latest build available, feel free to copy it and bookmark it.)