A three-phase induction motor has three windings, usually connected in either delta or ungrounded-wye. If the motor is operating properly, the current in all three windings (and thus, in the three phase conductors serving the motor) are IDENTICAL. Thus if you want to monitor the motor current you need only monitor ONE phase (i.e., measure the current on one of the three motor phase conductors).
Now if you want to calculate the POWER (real, reactive and/or apparent) consumed by the motor, it's considerably more complicated, and you probably want to use a 3-phase power meter intended for that purpose. Measuring RMS current alone, however, is much simpler.
For the CT (for however many phases you want to meter, though as I said above you only need one), the CT needs to be designed to handle the full-load current of the motor (meaning, when the motor is delivering rated horsepower at the shaft). The manufacturer rates this as Full-Load Amperes, or FLA. For the CT, the motor operating VOLTAGE doesn't matter, only the FLA, since all the CT cares about is current, rather than voltage (technically the insulation on the CT enclosure needs to be rated for at least the motor operating voltage, but this is usually much higher than the operating voltage of most commercial motors).
CT's are typically rated as N:5, where N is the rated primary current (in amperes) that the CT is designed to handle without going into saturation, and 5 is the number of amperes the CT sources from its secondary terminals, when the primary conductor is carrying rated current. For example, with a 300:5 CT, the secondary current will be 5/300 = 1/60 of the primary current that produced it. Thus 36A of primary current (i.e., on one of the motor phase conductors), for example, will produce 6A on the secondary of the CT - this is the current your metering circuit must be designed to handle and measure,
Whatever circuit you design to meter this current, needs to take care of this conversion factor (known as the CT Ratio), so that you're reading a number that corresponds to the current in the motor phase conductor(s), rather than on the secondary of the CT. All this means is your metering circuit multiplies wvatever current it's actually reading, by the CT ratio, so that the current value displayed, is the current in each of the motor windings. Once again, the current is the same in all three phases, so whatever you're measuring on one phase, is the same current that's flowing in the other two phases.
Hope this helps clarify the power systems side of your question.
