If you switched from three wire to one wire, you now have a two wire plug that is not connected. One of those wires goes to the generator light in the dash. When that wire is disconnected the generator light will come on, which is exactly what its supposed to do.
One wire alternators also kick in based on RPM only, since they do not have a voltage sense wire (the other wire you disconnected). The alternator may not kick in at idle, but once it does get enough RPM the power should keep up.