How to Loop and Combine Three Strings in Java

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public class Solution {
  public static String tripleTrouble(String one, String two, String three) {
    StringBuilder sb = new StringBuilder();
    
    for (int i=0; i<one.length(); i++) {
      sb.append(""+one.charAt(i)+two.charAt(i)+three.charAt(i));
    }
    
    return sb.toString();
  }
}

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import java.util.stream.Collectors;
import java.util.stream.IntStream;
public class Solution {
    public static String tripleTrouble(String one, String two, String three) {
        return IntStream.range(0, one.length())
           .mapToObj(i -> "" + one.charAt(i) + two.charAt(i) + three.charAt(i))
           .collect(Collectors.joining());
    }
}

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import org.junit.Test;
import static org.junit.Assert.assertEquals;
import java.util.Random;

public class TripleTests {
  private String alph = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
  private Random randGen = new Random();

  @Test
  public void basicTests() {
    System.out.println("Hard Coded Tests");
    assertEquals("abcabcabc", Solution.tripleTrouble("aaa", "bbb", "ccc"));
    assertEquals("abcabcabcabcabcabc", Solution.tripleTrouble("aaaaaa","bbbbbb","cccccc"));
    assertEquals("brrueordlnsl", Solution.tripleTrouble("burn", "reds", "roll"));
    assertEquals("Supermans", Solution.tripleTrouble("Sea", "urn", "pms"));
    assertEquals("LexLuthor", Solution.tripleTrouble("LLh","euo","xtr"));
  }
  
  @Test
  public void randomTests() {
    System.out.println("Testing 100 random inputs...");
    for (int i = 0; i < 100; i++) {
      int rand = randGen.nextInt(100);
      String one = randStr(rand);
      String two = randStr(rand);
      String three = randStr(rand);
      String msg = "Should work for:\n" + one + ",\n" + two + ",\n" + three + ".\n";
      assertEquals(msg, tripleAns(one, two, three), Solution.tripleTrouble(one, two, three));
    }
  }
  
  private String randStr(int n) {
    String t = "";
    for (int i = 0; i < n; i++) {
      t += "" + alph.charAt(randGen.nextInt(alph.length()));
    }
    return t;
  }
  
  private String tripleAns(String a, String b, String c) {
    String t = "";
    for (int i = 0; i < a.length(); i++) {
      t += a.substring(i, i+1) + b.substring(i, i+1) + c.substring(i, i+1);
    }
    return t;
  }
}