When I add third miniPCI card on routerboard 333, it is named wlan1 as the first card i insterted, so there are two cards with the same name. This is easily changed of course but as i recall from the past with 532 for example the third card was nicely named wlan3 (as expected). So, is this some kind of minor bug? 
or a feature maybe?
most likely that third cards was in that specific board a while ago with the name wlan1 and you took it out plugged another 2 cards which were names wlan1 and wlan2, then you plugged that third card and you got also wlan1 - it is becuase the RouterOS remembers the device names when it was plugged in for the first time.