Printing the max of groups of three random ints
The first thing about your code are the two lines inside your class Thing:

private Random r = new Random();
private int a = r.nextInt(), b = r.nextInt(), c = r.nextInt();


If Random r is used just to initialize the array and not in other methods inside the class , it is better to use it in the costructor of the class :

public Thing() {
    Random r = new Random();
    this.arr = new int[] {r.nextInt(), r.nextInt(), r.nextInt()};
}


You can check I initialize directy here the array of ints instead of defining variables a, b, c.
A good thing is also override the String method to print the internal state of a object:

@Override
public String toString() {
    return Arrays.toString(arr);
}


Now you can print the state of your Thing object in this way:

for (int i = 0;i &lt; 10; ++i){
    things[i] = new Thing();
    //print 3 numbers in each Thing object
    System.out.println(i + ": " + things[i]);
}


Your iterations with Stream are ok, but instead of use Stream because you are working with int values you can use instead IntStream simplifyng the code:

Stream.of(things).forEach(t -&gt; System.out.println(Arrays.stream(t.getArr()).max().getAsInt()));


Below the code of class Thing including all my modifies:

package codereview;

import java.util.Arrays;
import java.util.Random;
import java.util.stream.Stream;

public class Thing {

    private int[] arr;

    public Thing() {
        Random r = new Random();
        this.arr = new int[] {r.nextInt(), r.nextInt(), r.nextInt()};
    }

    public int[] getArr() {
        return arr;
    }

    @Override
    public String toString() {
        return Arrays.toString(arr);
    }


    public static void main(String[] args) {
        Thing[] things = new Thing[10];
        for (int i = 0;i &lt;10; ++i){
            things[i] = new Thing();
            //print 3 numbers in each Thing object
            System.out.println(i + ": " + things[i]);
        }
        System.out.println();
        Stream.of(things).forEach(t -&gt; System.out.println(Arrays.stream(t.getArr()).max().getAsInt()));
    }  
}