In any NEW installation, you must use a 3-wire-plus-ground cable and a four prong receptacle. When installing the pigtail cord, you must ensure that the neutral and ground/frame are NOT bonded together.
In the case of a built-in range, stovetop, or oven unit, the principle is the same; ONLY if the existing wiring only has two hots and a ground may the neutral and the ground/frame be connected; otherwise (existing 3 wire plus ground or new installation) you must have separate ground and neutral running to the appliance.
In no case in current code is combined neutral and ground acceptable EXCEPT for existing wiring and service entrance conductors.
In the case of the hot tub, I suspect that the existing wiring either was not originally for a hot tub or was incorrectly installed. It should be supplied with a 3-wire-plus-ground 120/240V service and GFI protection.