...whilst FAA and most of the rest of the world only requires 2.
Wrong.
With electric ADC's employed for each pilot, three are required, the one (standby) being directly connected to the static system, and therefore normally uncorrected for position/static system errors.
However, that 'third' altimeter (the standby) is normally connected to 28vdc (normally, essential/standby power), for a very important reason.
The internal vibrator, which acts to keep the internal mechanism properly...indicating.
And, yes, the forgoing is also a requirement, for quite a long time.
FAA and (nearly) all others.