Why There Is Voltage Drop In Particular 2 Phases Out Of 3 ?

Based upon the limited information provided, the 6.5% difference in voltages may be due to normal load imbalances and/or the effects of the harmonics from the non-linear DC drives. If all the loads are perfectly balanced across all three phases and the phase imbalance is a relatively new concern, then I would start with a thermal scan of all the switchgear, transformers, breakers, motors, etc.

Pay particular attention to all the connections and termination points as all three phases should be within a few degrees of each other; any hot spot or large variance would suggest a loose connection that must be corrected (with the circuit de-energized please!).

As RAM said, do a thermal scan and look for hot spots.

Did you measure the currents in the three phases, and if so, are the phases with the largest voltage drop carrying more current? What is the conductor size and what is the expected voltage drop with the phase currents? Does this explain your voltage drop?

Are these DC drives three phase? If not, that could explain the imbalance.

If they are three phase drives, it's possible that one or more diodes have failed to the open condition within one or more drives. Since they are DC drives, they are probably used for a variable speed application. A three phase DC drive commonly has 6 or 12 diodes. A single diode failed might not be noticed if the motor never needs full speed, but it would place an imbalance on the lines.

1. Load imbalance is one possibility.
2. Imperfect cable and terminal jointing is another.
3. Insufficient cable size is a third.
4. Partially damaged cables is a fourth.

If in doubt, consult a qualified high-voltage Electrical Engineer.