What is the power of a three phase circuit?
Neither loads nor generators "have to have" symmetric or equal electromotive forces, but it is desirable for economic utilization of equipment for the loads and sources to be symmetric. It is also less complicated to calculate power is everything is symmetric. The formula cited in the question applies only to balanced (symmetric) systems.

If the load or source is not balanced, the two-wattmeter method can be used to measure the power. The two-wattmeter method can be employed to measure the power in a 3 phase, three wire, star or delta connected, balanced or unbalanced load. The total power absorbed by the three loads, is equal to the sum of the powers measured by the two wattmeters.

The method of symmetrical components is used to simplify the analysis of three-phase unbalanced systems. The method involves converting a three-phase unbalanced system into two sets of balanced phasors and a set of single-phase phasors. These sets of phasors are called the positive-, negative-, and zero-sequence symmetrical components.

The two-wattmeter method and the method of symmetrical components are derived and described in detail in course notes and reference material that can be found on the internet as well as in text books that can be found in libraries.