Check for live high voltage shielded three phase conductor
Assuming the cable is unshielded, if you are close enough to it neither the magnetic field nor the electric field would cancel. The fields of the closest conductor would dominate. 

A small (~1cm long) conductive surface comparable in size to the conductor cross-section a few millimeters away from one of the conductors should have a capacitance of more or less 1pf to it. At 50Hz this is an impedance of \$\approx 3 G\Omega\$ quite high but not a problem. 

On a \$3M\Omega\$ input impedance (lower than most multimeters and high-impedance op-amps) you should still have a very measurable ~1V signal from your 1000V conductor. 

Place a short jacket with three conductive surfaces that are located 120º from each other, and you should be able to pickup each individual phase just by rotating the jacket until it is aligned with the conductors in the cable.