the nanny needs to set appropriate limits in a positive manner before the behavior escalates, create routines, have clear expecactions , and have pre-set consequences when the expectations aren't met. It should be a cooperative effort between the parent and the nanny so that the nanny knows what you are comfortable with/expect and so the boys know the same. If the nanny can't implement what has been agreed on then you need to find someone who can.

OP, there are only two reasons a nanny wouldn't be able to manage three young boys: either she is incompetent or you do not have enough rules/structure in place for your kids/haven't authorized the nanny to discipline them properly.