Three voltmeters are connected as shown 

A potential difference has been applied between A and B. On closing he switch S, readings of voltmeters?

Initially, when switch is open, $V_{2}and{V}_3$ will be out of circuit or they are short-circuited. So no potential difference is across $V_{2}and{V}_3.$On closing the switch, all three voltmeters will be in parallel. Now, there is some potential difference across $V_{2}and{V}_3.$ So potential difference across  $V_{2}and{V}_3$ increases and potential difference across  $V_1$remains the same.