The first example wouldn't qualify as an emergency in my book unless there was an indication of further damage or I thought the door might detach. For a basic gear door stuck open there's certainly be no need for the fire services to attend. I don't know about the 727 but as a trijet flying on two engines again it may be unnecessary to declare an emergency. I suppose that would depend on the circumstances of the failure/rundown.The last example is, again, marginal. The low oil pressure is a single indication. In the absence of other indicators it may be that the sensor has failed rather than the oil system. In the first and last cases it may be that an emergency is declared in order to receive special handling from ATC rather than in anticipation of any sort of problem on the ground. At my home base (LHR) officially you should declare an emergency if you require special handling from ATC and that will send the airport fire services to a local standby whether or not the pilots think they need their assistance.