public class Test
{
    public static void main( String[] args )
    {
        StringBuilder myName = new StringBuilder("AD-BJR5U");
        myName.setCharAt(3, '*');
        myName.setCharAt(4, '*');
        myName.setCharAt(5, '*');
        System.out.println(myName);
    }
}

(?<=^(?:.{3}|.{4}|.{5})).

import java.util.regex.Matcher;
import java.util.regex.Pattern;

final String regex = "(?<=^(?:.{3}|.{4}|.{5})).";
final String string = "AD-BJR5U";
final String subst = "*";

final Pattern pattern = Pattern.compile(regex);
final Matcher matcher = pattern.matcher(string);

// The substituted value will be contained in the result variable
final String result = matcher.replaceAll(subst);

System.out.println("Substitution result: " + result);

myName.replace(3, 5, "***");

// Keeping 3 firsts char; will replace the 3 of the center, and keep all to the end
^(.{3}).{3}(.*)$

str.replaceAll("^(.{3}).{3}(.*)$", "$1***$2");