Step down 3 phase 2kV AC to 3-12V at modest current (5-20mA max)
How about a 0.01uf capacitor, feeding a 1n756 zener diode. Or the 3-phase version.

1 farad at 1Hz is 0.16 ohms. 0.01uf at 1Hz is 16 million ohms.

0.01uf at 60Hz is 250,000 ohms, or 4uA per volt.

0.01uf at 60Hz at 2,000 volts is 8,000 ua or 8 milliamps

Seems that 3 high voltage caps, each in series with a high voltage diode, the three diode cathodes all feeding a zener diode ----- may suffice.

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Warning --- for this to work, the capacitor must be fully charging and then discharging, and then fully charged to the opposite polarity, on each cycle of the power.

That is why the zener diode works: the zener clamps to +5.6 volts on the (+) half of cycle, and then clamps to -0.7volts on the (-) half swing.

For this this to work, need 3 caps and 3 zeners and then 3 diodes from the top of each zener to a common capacitor, then to regulator.

For 1 volt of ripple at 10mA load, (given 8,333uF is needed for 1 volt at 1amp at 120Hertz recharge rate), your 360Hertz recharge rate and 1/100th the Iload allows a tiny cap ---- 33uf or 39uf.