The best way to do the color for the buttons is to create custom drawables for you button when pressed/active/disabled/focused (in my example I use button_faded.png, button_down.png & button_up.png) then define an xml to tell the button how to use it:
button.xml:
<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
<item android:drawable="@drawable/button_faded"
android:state_enabled="false" />
<item android:drawable="@drawable/button_down"
android:state_pressed="true" />
<item android:drawable="@drawable/button_up"
android:state_focused="true" />
<item android:drawable="@drawable/button_up" />
</selector>
Then in your layout set the background to "@drawable/button"
or better yet in your styles.xml create a button style:
<style name="Button">
<item name="android:layout_width">fill_parent</item>
<item name="android:layout_height">wrap_content</item>
<item name="android:textColor">#000000</item>
<item name="android:textSize">19sp</item>
<item name="android:background">@drawable/button</item>
<item name="android:gravity">center</item>
<item name="android:paddingLeft">0dp</item>
<item name="android:paddingRight">0dp</item>
<item name="android:paddingTop">5dp</item>
<item name="android:paddingBottom">5dp</item>
</style>
and in your layout xml tell the button to use the style:
<Button style="@style/Button" />
As far as the layout of the buttons... if you are looking for 1/3 of the screen each, create a horizontal LinearyLayout that contains all three buttons. Set the buttons width to fill_parent and give them all the same layout weight of let's say '1'... This should fill the row with the 3 buttons each the same size. You can then adjust the space between them using the layout margins or padding.