If the voltage at 3000 rpm is not above 13 volts, then you have a problem with the electrical generating system.
The Stator generates up to 70 volts AC, the rectifiers convert the AC to DC voltage, and the regulator prevents the DC voltage from getting above 14.5 volts.
Of course, there are wires and connectors involved. So, you need to find out which of the parts is not functioning.
NOTE: The following is a general idea for solving the problem and may not work with your system..... Get a Service manual or take to a mechanic.
Be sure to clean all of the connectors and make sure the ground wires/bolts are rust-free and secure before doing any additional evaluations.
You can unplug the stator from the rectifier and measure the AC voltage.
You can unplug the rectifier and use an Ohmmeter to measure the resistances of the diodes.
Almost impossible to evaluate the voltage regulator unless you have an electronics shop, so evaluate the stator & rectifiers first.
If they are good, then the regulator is probably the problem.
Scoot Safely,