Well, I don't know about other Lidl's, but if a queue is too long for 2 checkouts, one of the 2 cashiers will ring the bell three times so cashier no. 3 knows he or she should open a third. That's what I was taught, to just ring the bell for another cashier to open a next checkout if we think the queue is too long for us to handle fast enough.
Oh and sometimes, depending at which checkout you're sitting, it's very difficult to see how far the queue is stretching.