The simplest solution would be using empty views with weights as separators.
<LinearLayout android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="horizontal">
<!-- Left button -->
<Button ...
... />
<View android:layout_width="0dp"
android:layout_height="match_parent"
android:layout_weight="1" />
<!-- Middle button -->
<Button ...
... />
<View android:layout_width="0dp"
android:layout_height="match_parent"
android:layout_weight="1" />
<!-- Right button -->
<Button ...
... />
</LinearLayout>
The separator views can be made invisible as an optimization, because they don't draw anything and are used only for layout. You can tweak the actual 'layout_weight' values to get the desired layout. Starting from API level 14 you can use instances of Space as separators which will improve performance and readability (there is also a version of Space in the support library).