API v3 get multiple results for one field
Contact API sends one result per contact. In case if you want to retrieve all custom field value(multiple) of a contact than you need to use CustomValue api. Snippet for reference

REST:

http://localhost/drupal7/sites/all/modules/civicrm/extern/rest.php?entity=CustomValue&amp;action=get&amp;api_key=userkey&amp;key=sitekey&amp;json={"sequential":1,"return":"custom_81,custom_80","entity_id":296,"entityType":"contact"}


PHP

$result = civicrm_api3('CustomValue', 'get', [
  'sequential' =&gt; 1,
  'return' =&gt; ["custom_81", "custom_80"],
  'entity_id' =&gt; 296,
  'entityType' =&gt; "contact",
]);


entity_id = contact id

return = custom fields

HTH

Pradeep