gistfile1.txt

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <Button
        android:id="@+id/one"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        app:layout_constraintVertical_bias="0.2"
        app:layout_constraintBottom_toTopOf="@id/two"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_chainStyle="packed"
        app:layout_constraintWidth_default="percent"
        app:layout_constraintWidth_percent="0.7" />

    <Button
        android:id="@+id/two"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintBottom_toTopOf="@id/three"
        app:layout_constraintLeft_toLeftOf="@id/one"
        app:layout_constraintRight_toRightOf="@id/one"
        app:layout_constraintTop_toBottomOf="@id/one" />

    <Button
        android:id="@+id/three"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="@id/one"
        app:layout_constraintRight_toRightOf="@id/one"
        app:layout_constraintTop_toBottomOf="@id/two" />
</android.support.constraint.ConstraintLayout>