Try this..
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent" >
<LinearLayout
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerInParent="true"
android:orientation="vertical" >
<Button
android:id="@+id/multiplay"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:background="@drawable/menubutton"
android:text="@string/multiplayer" />
<Button
android:id="@+id/options"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:background="@drawable/menubutton"
android:text="@string/options" />
<Button
android:id="@+id/singleplay"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:background="@drawable/menubutton"
android:text="@string/singleplayer" />
</LinearLayout>
</RelativeLayout>