As you’re filming, select the Camera icon in the left hand menu. This will bring up the Invite Camera screen again.
Remove the camera that is not sending signal and add another camera from your list of available camera ops (it is always a good idea to have a standby camera person or 2, loss of battery power or lack of internet connection can cause camera outages).
Once the new camera is invited select the broadcast icon again and continue your shoot. Once the new camera begins to send signal it will populate the Director interface and you can then continue your shoot with 3 cameras.