How to connect 3-wire appliance to 4-wire junction box
That is correct wiring. You are in the very good situation of: Had 4-wire, now only need 3-wire. Far more common is either: &quot;Had 3-wire, now need 4-wire&quot; or &quot;Had 3-wire that should have been 4-wire but grandfathered&quot;.
The only issue is current requirements. Depending on the size of the existing wires and breaker, you may be in any of the following situations:

Everything OK as-is
Wire OK but need to put in a larger breaker
Wire OK but need to put in a smaller breaker
Need to put in larger wire and a larger breaker.

Can't tell which of these 4 situations applies without knowing the specs of the new cooktop, the size of your existing wire and the size of your existing breaker.