use a android:weightSum="3"
for your inner
Layout with the three elements, and after that,
android:layout_weight="1"
to each view
this will make the views to hold 1/3 of the available space
<LinearLayout
android:id="@+id/linearLayout2"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:orientation="horizontal"
android:background="@android:color/holo_blue_light"
android:weightSum="3"
>
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Subtotal:"
android:layout_weight="1"/>
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textColor="#800000"
android:text="£???"
android:layout_weight="1" />
<Button
android:id="@+id/checkout"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:gravity="right"
android:text="Checkout"
android:layout_weight="1">
</Button>
</LinearLayout>
EDIT fill_parent is deprecated use match_parent