Here like you requested

import java.util.Scanner;

//Description Write a program that calculates the average of
//three user test scores, then display the average and a letter
//grade that corresponds to the average.

//Need to: Implement full validation of user input to
//ensure than inputs are not negative; the validation routine should be
//written as a method.

//I changd the code like you requested btw you might wanna correct
//the average to 2 decimal else you will have outputs like this: "Your average is 76.66666666666667 or C"
//also this assumes that the input is a grade between 0 and 100 if the input is not always like
//that then pm me i can write it then if you want but try it yourself first
//also if the input is not a double or int or something in its scope like String, char, boolean it will
//throw an error if you wanted that changed to an msg pm me --verd
public class TestAverage {

   public static void main(String[] args) {

       double score1,score2,score3,
              average;

       score1 = scannerInput("Enter score 1:");
       score2 = scannerInput("Enter score 2:");
       score3 = scannerInput("Enter score 3:");

       average = (score1 + score2 + score3) / 3;

       if(average >= 90){
           System.out.println("Your average is " + average + " or A");
       }
       else if(average >= 80){
           System.out.println( "Your average is " + average + " or B");
       }
       else if(average >= 70){
           System.out.println("Your average is " + average + " or C");
       }
       else if(average >= 60){
           System.out.println("Your average is " + average + " or D");
       }
       else if(average < 60) {
           System.out.println("Your average is " + average + " or F");
       }


   }
   public static double scannerInput(String msg) {
      System.out.println(msg); //prints msg
      Scanner s =new Scanner(System.in); //creates scanner object
      double temp = s.nextDouble(); //gets double

      while(0>temp || temp>100) { //validation (if temp is lower than 0 OR if temp is higher than 100)
          System.out.println("The grade must be between 0 and 100!");
          temp = s.nextDouble();
      }

       return temp; // returns temp
   }

}