Keep in mind that the VR tubes are voltage regulating, but also current limited. If I recall correctly, those 0A3 tubes are 75V, meaning 3 in series will typically regulate at 225V. However, 0A3 tubes are only able to handle about 40 mA of current singly or in series, so it's usage is limited to lower current applications.
https://frank.pocnet.net/sheets/049/0/0A3.pdf