I purchased a slide in range with cooktop. I am putting in where old cooktop was. I only have 3 wires, 1 balck, 1 white, 1 ground. The only range outlet I could find was a four prong. I thought black was hot and white was neutral and then of course you have the ground. I am finding out now I am supposed to have 4 wires. What do I do now?

You should check the voltage of the existing black and white, I am sure you will find 240 volts. While the white is dedicated to be neutral, it is also allowed to be used as a hot for 240 volts. Unlike back then, now the white must be marked as hot when used as a live wire.

Back then one wire was allowed to be used as a neutral and equipment ground for appliances. Now for new installations 4 wire is required, separating the neutral and ground. However, since the cable is existing, it may remain.

If possible and practical, installing a new 4 wire cable is always better, but you may continue to use the 3 wire cable you have.

Once you prove the black and white is the 240 volts, the remaining wire will be both the neutral and equipment ground. The new cooktop will have a white and a bare or green, which will connect to the feeder neutral/ground wire.