public class Problem {
private static final int MY_THREADS = 10;


public static void main(String[] args) {
    double[] array = {...};

    double[] maximums = new double[3];

    for (int i = 0; i < maximums.length; ++i) {
        maximums[i] = Double.MIN_VALUE;
    }

    ExecutorService executor = Executors.newFixedThreadPool(MY_THREADS);

    Runnable worker = new MyRunnable(array, maximums);
    executor.execute(worker);
    executor.shutdown();

    while (!executor.isTerminated()) {

    }
    System.out.println(Arrays.toString(maximums));
}

public static class MyRunnable implements Runnable {
    private double[] array;

    private double[] maximums;

    MyRunnable(double[] array, double[] maximums) {
        this.array = array;
        this.maximums = maximums;
    }

    @Override
    public void run() {

        int i = 0;
        while (i < array.length) {
            if (array[i] > maximums[getMinIndex(maximums)]) {
                maximums[getMinIndex(maximums)] = array[i];
            }

            ++i;
        }
    }
}

private static int getMinIndex(double[] array) {
    int minIndex = -1;
    double min = Double.MAX_VALUE;

    for (int i = 0; i < array.length; ++i) {
        if (array[i] < min) {
            min = array[i];
            minIndex = i;
        }
    }
    return minIndex;
}