Converting KFDD500ESS Dual Fuel Range from 4 wire to 3 wire plug
If the range does not have instructions for wiring with a 3-wire plug with hot1-hot2-neutral, then improvising would be out of the question, both for actual safety and for satisfying the code.

Without the range installed it is not that difficult to remove the receptacle and put in a 4-wire 50-A receptacle (NEMA 14-50). You can fit a 40-A or a 50-A breaker depending on what the wire allows, but can fit a 40-A breaker even with #6 wire. You would add a #8 or #6 copper ground wire (to match the existing conductors) going back to the panel. Or if it is available, connect by tap onto the  heavy, uninsulated ground in your attic that goes back to the panel. Use a split bolt connector to tap. The added ground does not have to follow the route of the existing conductor.

First examine the contents of the box. Of course, shut off the breaker, then pull out the existing 3-wire receptacle and see what size wire you have. How many conductors do you have? Probably just three, but maybe you'd get lucky and find an unused ground coiled up in the box.


NEMA 14-50 

EDIT

You should at this point contact your local electrical code enforcement and ask them about the approach outlined above. If this is allowed, then it would not be dead easy to fish a ground down into the wall and into the existing box. Someone here would have to tell you the practicalities on how to do it. What may happen here is that you will learn from the experts here what is a reasonable arrangement, and if you have never done this before and need it done without delay, then you might have to hire an electrician, but at least you would have in mind your instructions to the electrician.