3 wire cooktop (réduit black & grounds) to a 4 wire ourlet (red black wite & ground)
Connect red, black and green/bare (ground.) Put a wire nut on the unused white in the wall box, in case a different appliance is used later that needs it.
Or: If using a plug into a 4-wire receptacle, just don't connect to the neutral (white) blade of the plug.