How do i make a 3 phase generator for a wind turbine?

The first thing to know is what is its purpose. What power is involved. It is not likely you can make one from scratch, rather modify an existing one. The simplest case for modification: the alternator in a car is a three phase alternator, with diode rectifiers to produce DC. There are many articles about these on the internet. The rotating field is powered from the battery initially to get it started. This needs to be considered with a wind generator in the context of power used on standby. The output voltage is around 14V when regulated as a DC voltage to charge a battery. The power is of the order 600 watts. This power suits small 2 or 3 meter diameter wind turbines. The frequency and voltage will depend on the speed. The current available depends on the available torque from the turbine. The power of the turbine is related to speed and torque, which correspond with power of the generator voltage and current. The rpm of the turbine varies a lot, and the generator should be made to match this speed range. The output power of a wind generator is disappointingly low unless the wind is strong. There is a threshold effect too. At high wind speeds special precautions are needed to protect the generator. In some cases car alternators are rewound to provide a more suitable speed and voltage. The regulator is modified, and sometimes the field is replaced with permanent magnets (so regulation is lost, but no standby current). They are normally used for battery charging, and the AC power is provided using an inverter. The best way to get an AC supply is to use an inverter. The generator produces DC and an electronic inverter provides accurate frequency, waveform and voltage regulation. These are not simple. An inverter designed for grid connection is more complex still and the design needs to be approved generally. Large generators use gears, blade control and electronic inverter technology associated with an induction generator. These are quite complex designs. You can research the points mentioned on the internet - this is a complex subject, but there are designs people have tried for small wind generators. There is not enough power in small wind turbines for a typical 3 phase load. Big ones are only suitable to connect to a grid. There needs to be on line backup power.

The question is rather vague. 'Wind generator' is something to produce wind. I believe you mean a generator driven by a wind turbine. If you have the qualifications of an Electrical and Turbine Engineer, then with the correct manuals, the construction should be possible....with an awful lot of expertise and help.

Most car alternators have a three phase output but it's rectified at source. So the short version of the above is a car alternator will provide a 3 phase AC output without the internal diodes in the way.