关于布局文件layout_gravity的问题

我想在屏幕底部放3个imagebutton，做成左中右的布局，但是在tablerow中layout_gravity不起作用，
下面布局文件<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:background="#606060"
    android:orientation="vertical" >    <RelativeLayout 
        android:id="@+id/head"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        >
    <ImageView 
        android:id="@+id/title"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:adjustViewBounds="true"
        android:contentDescription="@string/title"
        android:src="@drawable/play"
        />
    
    <ImageButton 
        android:id="@+id/playOrPause"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:contentDescription="@string/music"
        android:adjustViewBounds="true"
        android:layout_gravity="right"
        android:layout_toRightOf="@id/title"
        android:src="@drawable/play"
        />
    </RelativeLayout>
    
   <FrameLayout 
       android:id="@+id/middlecontent"
       android:layout_width="fill_parent"
       android:layout_height="wrap_content"
   android:layout_weight="1"       
       >
       
       
   </FrameLayout>
   
   
    <TableLayout
        android:id="@+id/flip"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        >
   
    <TableRow 
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:orientation="vertical"
        >
        
        <ImageButton 
            android:id="@+id/left"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:src="@drawable/left"
            android:adjustViewBounds="true"
            android:contentDescription="@string/leftye"
            />
        
        <ImageButton 
            android:id="@+id/goContent"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:adjustViewBounds="true"
            android:layout_gravity="center"
            android:contentDescription="@string/middle"
            />
        
        <ImageButton 
            android:id="@+id/right"
             android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:adjustViewBounds="true"
            android:src="@drawable/right"
            android:layout_gravity="right"
            android:contentDescription="@string/rightye"
            />
       
        </TableRow>
        
    </TableLayout>
</LinearLayout>