How can I measure ampere flow on the 3phase 3 core (RYB) cables with clamp tester, where I cant separate the phases to measure it.
If you can't separate the phases you can't measure it with a clamp. The current it will see is the sum of the currents in each of the phases, and that will be zero. (At best you can measure the neutral's difference current this way.)
Never say never !
With very great difficulty :-)
Hard: A Hall sensor placed as close as possible to one phase and as far away from the other two will be differentially affected by the phases with the close one predominating. You would need to calibrate both for effect of desired phase AND for the unwanted effects of the other two. Minmising proximity to the desired phase is a major aim.
Assume that distance to the two unwanted phases is 6mm.
Pressing a Hall sensor or magnetic concentrator hard against the cable sheath in parallel with the conductor and symmetrically away from the other two conductors should give best result.
If the conductors spiral (quite likely) then making a guide of magnetically permeable material which tracks the spiral will probably help.
If you have three probes that can be symmetrically placed for optimum input from each conductor you can solve 3 equations ofr a notionally perfect solution . As this requires complete symmetry you will only approach a perfect result.
If pickup from target phase is 1 unit and pickup from other phases is k units k < or << 1 then.
Reading 1 = proportional to I1 + K(I2 + I3)
Solve for 3 currents.
Very very hard (probably): The terminally enthused might try measuring temperature rise above each conductor.
Very hard (maybe): If you can apply slight modulation to the voltage at some nearby point you may be able to determine phase current by the effect of the modulation on magnetic field strengths. Or not :-).
If you can access the voltage readings at two points along the cable you can measure voltage drops.
Just maybe a contact electrostatic meter against the outside of the cable will allow this at two points non-contact. Liable to very very hard.