3.3V is about right for the filament voltage but it has to be measured across pins 2 and 3, not to ground and you should measure it with the display connected.
In this kind of display, the filament is also the cathode connection, they work like vacuum tube triodes where the anode has to be more positive than the cathode for conduction to take place. In this case, the anode is probably ground and the cathode is made negative instead.
If those voltages are there, your problem is elsewhere in the unit. It seems the power supply is working OK.
Brian.