For a three-phase balanced system, the instantaneous power over three-phase can be derived as a constant term equal to (3*E*I*cos(phi)). Again, the (E*I*cos(phi)) is the average value of the active power as can be derived in a single-phase circuit. Then also, what is the need of a shunt-type reactive power compensator at the load end in a scenario where a balanced three-phase voltage source is supplying a balanced three-phase current load.
Generally, reactive power causes an additional current, and the power lines, power distribution system, fuses, etc. have to be designed to deal with the resulting current. So, the purpose of compensators is to keep this additional current reasonably small (about cos(phi) > 0.94).
"Balanced" means that the sum of the three currents is zero. E. g. if you connect three identical capacitors to the three phases, you would have a balanced load but cos(phi) = 0, for ideal capacitors. The same applies to ideal inductors. I. e., active power = 0 but large currents might be flowing through the power lines.
In short, compensation is necessary in three-phase systems for the same reason as in single-phase supply.
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