Calculate all custom field values in the post loop

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;