Try this:
//loop start
$total_amount = get_post_meta($post->ID, \'custom_field1\', true);
$total_amount += get_post_meta($post->ID, \'custom_field2\', true);
$total_amount += get_post_meta($post->ID, \'custom_field3\', true);
//loop end
echo \'Total: \'.$total_amount;