Finding technical manuals like the DMBOK on GitHub is often a red flag for two reasons:
If you are an enterprise user, your organization may already have a license. Many large corporations purchase multi-user licenses for their DAM Center of Excellence teams. Ask your internal knowledge management or IT department. damadmbok 3rd edition pdf github
Since the 3rd edition is not yet widely released (or is in very early release), your best bet is to monitor: Finding technical manuals like the DMBOK on GitHub