If you are measuring the voltage of a three phase power system you take only two phase in account then why there is a need for third phase?

If you are measuring the voltage of a three phase power system you take only two phase in account then why there is a need for third phase?
Answer Take all phases into account. Voltage is measured betweentwo phases of the three phases at one time, so what this means isthis...first you read voltage between line 1 and line 2...then youread voltage between line 2 and line 3...and then from line 1 toline 3. Each phase of a 3 phase system is 120 degrees from theother in a 360 degree pattern. It takes all 3 phases to start a 3phase motor but can run on two. If a 3 phase motor tries to starton two phases it is refered to as single phasing and can damage themotor.
Another Answer

First of all, let's get the terminology correct. The wires thatjoin a three-phase load to its supply are called 'LINEconductors', not 'phase conductors'! This is veryimportant. Phases, which are normally inaccessible, are either thegenerator windings, the transformer windings, or the individualloads, connected to the line conductors -this can make measuringphase voltages very difficult unless you can access the interior ofthese machines/loads.

The voltage of a three-phase system is normally defined in terms ofits line voltage, not its phase voltage, so onenormally measures its line voltage by connecting a voltmeterbetween any two line conductors or terminals. As the line voltagesare determined by the supply system, all line voltages should bethe same, regardless of which line conductors you choose to placethe voltmeter between.

The important thing, however, and this is something your voltmeterwill NOT tell you, is that the three line voltages are out of phasewith each other -each lagging its predecessor by 120 electricaldegrees. And this is important, because it is the phasedisplacement between these voltages, not the magnitude of eachvoltage, that allows -for example- a three-phase motor toself-start.