How do filesystems cope with the apparent capacity of the disk changing? If you put a lot of data on that compresses really well, suddenly your disk will appear to have become larger.
The filesystem ontop always sees 3TB, unless you explicitly modify the VDO device. Of course, you have to monitor the VDO status tightly: if you happen to store data on the filesystem which is absolutely unique, has no zeros and is uncompressable, then dedup/compression can not do anything. The VDO device can only consume up to ~1TB of such data. Your monitoring should detect the low space on the VDO backend before, and you should then either stop writing or extend the backend device.
Should be tried out before relying on it. The current behaviour seems to be considered a bug, https://bugzilla.redhat.com/show_bug.cgi?id=1519377 has details.