Those are the Battery, Regulator/Rectifier (R/R) and the Stator.
The job of the Stator is to convert mechanical energy from the engine back into electricity. The output from the stator on most modern bikes is three phase (hence the 3 wires) alternating current at a peak voltage well over the 12-14V DC voltage that your electrical system requres.
The R/R unit converts the alternating current (AC) in the diode bridge rectifier section then regulates the voltage to limit it to around 14V DC where it is fed back into the Battery and powers the rest of the bikes electrics.The voltage is higher than the battery voltage so current flows from the R/R unit back to the battery to charge it.
Now on to testing the main charging function, the voltage output from the Regulator. Test the voltage with the bike running at fast idle, roughly between 2000 and 5000 rpm, to confirm if it is getting the correct amount of charge from the bike. With the Multimeter set to 20v DC place the meter leads across the Battery as before, if all is healthy you should be getting between 13.5-14.5 volts.
Rev the bike and the voltage should be at least in the 13’s at all engine rpm’s and go no higher than around 14.5v. If you are reading 15v or higher it is bad and means that the R/R is not regulating the extra voltage from the Stator and overcharging the Battery.
If you are getting a normal charging voltage but when you rev the bike harder the voltage drops, even down to zero, then the Regulator/Rectifier is breaking down under load and will need replacing.