Code for the ESP32 written in Arduino:

//define pin numbers
//to which the sensors are connected
#define BUTTON 13
#define KNOB A0
#define FLEX A1


void setup() {
    //set up pin modes to be inputs for reading
    pinMode(BUTTON, INPUT);
    pinMode(KNOB, INPUT);
    pinMode(FLEX, INPUT);
    //start serial communication
    Serial.begin(115200);

}

void loop() {
    //read the high-low state of the button
    //print it to the serial connection
    Serial.print(digitalRead(BUTTON));
    //print a separator symbol to the serial connection
    Serial.print(',');
    //read the position of the potentiometer in a range
    //print it to the serial connection
    Serial.print(analogRead(KNOB));
    //print a separator symbol to the serial connection
    Serial.print(',');
    //read the position of the flex sensor in a range
    //print it to the serial connection and
    //add the END-OF-LINE character to mark the end of transmission
    Serial.println(analogRead(FLEX));
    //short delay
    delay(10);
}
            

Code written in Processing:

// import the library containing the functionality
// necessary to establish and process serial communications                
import processing.serial.*;
// create a variable to represent a serial port
Serial mySerial;

float btn = 0;
float knob = 0;
float flex = 0;

void setup(){
    size(800, 800);
    // create a new serial connection and store its representation
    // in the variable created above
    // the connection is created between 'this' program
    // and the serial port with the name 'COM3'
    // (replace with a different name as needed - 
    // if you're using it with an Arduino, check the name of the port
    // to which your board is connected by switching to ArduinoIDE, 
    // then going to Tools -> Port submenu)
    // the communicaiton is established at 115200 bits per second
    // this rate needs to match the rate of transmission
    // (if you're using it with an Arduino, check the function
    // 'Serial.begin(rate_here)' - make sure the numbers match)
    mySerial = new Serial(this, "COM3", 115200);
    // accept characters until the special character is encountered
    // in this case '\n', signifying END-OF-LINE (EDL) character
    // This would correspond to the statement 'Serial.println'
    // in Arduino, which adds the EDL character automatically -
    // as opposed to 'Serial.print', which does not
    mySerial.bufferUntil('\n');
}

void draw(){
    //use the button to control the background
    if(btn < 1){ //if 0
        background(0);
    }
    else {
        background(255);
    }
    //use the flex to control the color
    fill(flex);
    //use the potentiometer to control the size
    circle(width/2, height/2, knob);
}
// this function is triggered every time the special character 
// which we passed to the 'bufferUntil()' function is encountered
void serialEvent(Serial p){
    // read the string accumulated since the end of the last transmission
    // and store it in a temporary variable
    String transmission = p.readString();
    // clean up the transmission removind whitespace characters,
    // including end-of-line character
    String cleanedUpTransmission = trim(transmission);
    // split the transmitted string using the separator character
    // (in this case a comma) and convert it into the array of floats
    // e.g.: "1,23,45" -> [1.0, 23.0, 45.0] 
    float[] values = float(split(cleanedUpTransmission, ','));
    // check to make sure all the values are present
    // in this case the expected number of values is 3
    if(values.length == 3){
        // reassign values to prepared variables
        // using their ordering to distinguish which is which
        btn = values[0];
        // convert the value stored as a String into a floating-point
        // number and re-map it to a usable range from the
        // original range between 0 and 4095 for ESP32 
        // (replace with 0 to 1023 for Arduino)
        // see https://processing.org/reference/map_.html
        knob = map(values[1], 0, 4095, 0, width);
        flex = map(values[2], 3030, 1600, 0, 255);
    }  
}