How to see #default_values in multiselect form
The default value needs to be the key of the options array, so assuming the rest of your code is correct, this should work:

$form['group_config']['tematicas'] = array(
  '#type' =&gt; 'multiselect',
  '#title' =&gt; t('Temáticas'),
  '#options' =&gt; $options3,
  '#required' =&gt; TRUE,
  '#multiple'=&gt;TRUE,
  '#default_value'=&gt; $term2-&gt;tid,
  '#prefix' =&gt; '&lt;p&gt;&lt;br&gt;',
  '#suffix' =&gt; '&lt;/p&gt;',
  '#size'=&gt;20,
);