I would like to know how teach my three years old son Islam. I am a Muslim and I do every thing I know about my religion. I live in Spain. I am writing from a pubic center. It is really hard and complicated task. I do need help.

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

For sure, it is difficult to teach Islam to a child who is three years old, since he is still too young to discern Islamic or general affairs or to understand meaning of words.
We have to teach him essentials and manners of Islam gradually, taking his age into consideration. In other words, we teach him such issues when he can comprehend them.
Here, we tell him about, Allah, The Creator, The Provider with the means of subsistence, The Great, The Only and One God.
We teach the child to attest to The Oneness of God by using his index finger.
In addition, we teach him that one who tells the truth, obeys Allah and his parents will enter Paradise, which is full of great blessings. We implant in him fear of the Hell-fire in case of telling lies.
Also, we teach him that he is a Muslim and should love Muslims and that there are non-Muslims.
We have to help him learn al-Fatiha (the opening chapter of the Qur'an) and some short Surahs.
Here, it is useful to state two points: (1) treat the child gently, (2) follow a gradual method while teaching him.
However, according to Shari'a, a guardian is not required to order a child to perform forms of worship except after attaining the age of discretion, i.e. seven years old. Thus, we restate that a three-year-child has not the ability of discretion to comprehend such matters.
Allah knows best.