A little help to a CF newbie for combine fields

Hi guys
i'm new on Computed Field and i need a little help 'cause i don't understand why a simply, simply code doesn't work .

I wanto to combine three fields and i set this code:
Computed Code:
$node_field[0]['value'] =
$node->field_advertiser[0]['value'] +
$node->field_advertiser2[0]['value']+
$node->field_advertiser3[0]['value'];
Display Format:
$display = $node_field_item['value'];

Doesn't display nothing...tried also :
Computed Code:
$node_field[0]['value'] =
$node->field_advertiser[0]['value'] .
$node->field_advertiser2[0]['value'].
$node->field_advertiser3[0]['value'];

But i can be able to display computed value

Can you help me ?
Thanks in advance