How does use an arduino like a A/D converter for convert 3 analogue signal from 3 poteciometer to a raspberry?
It is not clear what you mean by “digital port”. The simplest mean to
communicate from the Arduino to the Raspberry is to use the Arduino's
serial port, which gets into the Raspberry Pi through USB. Just like you
would do if you had a PC instead of the Raspberry:

/*
 * Continuously report the analog readings on pins A0 through A2.
 */

void setup() {
    Serial.begin(9600);
}

void loop() {
    Serial.print("0 "); Serial.println(analogRead(A0));
    Serial.print("1 "); Serial.println(analogRead(A1));
    Serial.print("2 "); Serial.println(analogRead(A2));
}


This program will continuously output a text stream like this:

0 145
1 220
2 865
0 142
1 223
...


where the first column is the analog channel number and the second
column is the ADC value. How you handle this stream on the Raspberry
side is up to you.