3-speed DC fan control with Uno
Solved simply by condensing code and getting rid of unnecessary variable declarations.

Corrected program:

    int motorPin1 = 3;
    int motorPin2 = 9;
    int DCfanSpeed = 0; 




    void setup() 
    { 
      pinMode(motorPin1, OUTPUT);
      pinMode(motorPin2, OUTPUT);
      digitalWrite(A0, INPUT_PULLUP);
      digitalWrite(A1, INPUT_PULLUP);
      digitalWrite(A2, INPUT_PULLUP);
      digitalWrite(A3, INPUT_PULLUP);
      Serial.begin(9600);
    } 


    void loop() 
     { 


       int voltsZero = analogRead(A0)/204.6; // OFF SWITCH
       if(voltsZero == 5)
        {
          delay(300);
          if(voltsZero == 5) DCfanSpeed = 0;

           while(voltsZero == 5) 
            {
              analogWrite(motorPin1, DCfanSpeed);
              analogWrite(motorPin2, DCfanSpeed);
              voltsZero = analogRead(A0)/204.6;

            }
         }

      int voltsOne = analogRead(A1)/204.6;    // LOW SWITCH
      if(voltsOne == 5)
        {
          delay(300);
          if(voltsOne == 5) DCfanSpeed = 128;
          while(voltsOne == 5)
           {
             analogWrite(motorPin1, DCfanSpeed);
             analogWrite(motorPin2, DCfanSpeed); 
             voltsOne = analogRead(A1)/204.6;
           }
        }

       int voltsTwo = analogRead(A2)/204.6;  //  MED SWITCH
       if(voltsTwo == 5)
        {
          delay(300);
          if(voltsTwo == 5) DCfanSpeed = 191;
          while(voltsTwo == 5)
           {
             analogWrite(motorPin1, DCfanSpeed);
             analogWrite(motorPin2, DCfanSpeed);
             voltsTwo = analogRead(A2)/204.6;

           }
        }

      int voltsThree = analogRead(A3)/204.6;  //  HIGH SWITCH
      if(voltsThree == 5)
        {
          delay(300);
          if(voltsThree == 5) DCfanSpeed = 255;
          while(voltsThree == 5) 
           {
             analogWrite(motorPin1, DCfanSpeed);
             analogWrite(motorPin2, DCfanSpeed);
             voltsThree = analogRead(A3)/204.6;
           }
         }

    }