How many wires used for 3 phase?

There is no voltage in three phase wire. The ability of wire to carry voltage is dependant upon the insulation that surrounds the wire. The thicker the insulation the higher the voltage potential can become. Three standard insulation voltages are 300, 600 and 1000 volts.

Another Answer

First of all, there is no such thing as a voltage 'on' a wire. 'Voltage' is another Word for 'potential difference', so a voltage can only exist between two wires. Voltages in three-phase systems are generally specified in terms of their line voltages-i.e. the voltage between any two line conductors. These depend on the electrical standards used in the country in which you live. In the UK, for example, three-phase transmission lines will have line voltages of 400 kV, 275 kV, or 132 kV, while distribution lines will have line voltages of 33 kV and 11 kV, and low-voltage distribution line voltage is 400 V.