Let me clarify a bit - I mean that I want the script to run 3,6,9,12,15,18,21 etc.. No need less than 3 months at all so I put key > 3.

Code:
raw USERNOTICE:*:{
  if ($msgtags(room-id).key == 12665) && (resub isin $msgtags) {
    if ($msgtags(msg-param-months).key > 3 ) && (3 // $msgtags(msg-param-months).key) {
      msg $1 !bonus $msgtags(display-name).key $calc( 500 + 100 *   $floor( $calc( ( $msgtags(msg-param-months).key / 3 ) - 1 ) ) )
    }
  }