New Electric Stove, Old Wiring
That old 3-prong receptacle is dangerous.  Upgrade to a 4-prong receptacle, by adding a ground wire.  

That dangerous socket

The old 3-prong NEMA 10 receptacle doesn't have a ground wire.  It bootlegs ground using the neutral pin - which is a terrible idea because any problem with the neutral wire will energize the chassis of your range.  Appliance makers lobbied NFPA to allow this for "grandfathered" work, on the rationalization that these plugs are rarely disturbed. 

The new 4-prong NEMA 14 receptacle includes a proper ground wire.  If neutral fails, the oven light simply goes out.  If neutral and ground fail, the chassis still will not be energized.  Much safer. 

Retrofit a ground

The NEC rules allow you to retrofit a ground wire separately, any way you can.  It does not need to follow the same route as the power cables.  It does need to go back to the the same breaker box, however. 

You don't need to go all the way to the breaker box if you can reach another circuit which has a ground wire that's thick enough (and is also served out of that same box). Your minimum ground wire size is 10 AWG, and a water heater, dryer or built-in A/C will have that if it has a ground.  Metal conduit is also a valid grounding path (make sure it isn't corroded, damaged or loosened).  If the wiring is already in metal conduit, you're done :) 

The cheapest way to buy 10 AWG ground wire is to actually buy 8 AWG bare copper wire, readily available at most hardware stores. 

After that, you buy a matching NEMA 14 receptacle (probably 14-50) and connect  


The two hots to the two hots obviously. 
The grandfathered neutral to neutral (it may be bare; if so, wrap it with electrical tape so it doesn't contact the ground wire; bonus points for style if you use white electrical tape). 
The proper ground wire to the ground terminal.  You don't want it touching neutral because that would defeat the safety benefit.   


Check the range to make sure it's jumpered correctly for 4-wire and not 3-wire, plug it in, turn the breaker back on and enjoy!

(by the way, once you have retrofit the ground wire, you can also use that to ground other ungrounded circuits served out of that same panel.)