How should I connect an electric cook top to an existing dededicated 3 wire armored cable?
No. The bare copper wire is the grounding conductor, and it should be connected to the circuits grounding conductor. 

If there is no grounded "neutral" conductor coming from the cooktop, you should cap the white wire off.  The grounded "neutral" conductor is only used for 120 Volt or 120/240 Volt circuits, and is not required for straight 240 Volt circuits like your cooktop requires.  

Read and understand the manufacturer's installation instructions, before you proceed further.