Buick,Failing to maintain proper voltage after replacing the starter the alternator and the battery

One week ago I replaced my starter that had failed since then I started having voltage issues so I had my battery tested and it was a bad battery I also had my alternator tested turned out to them it was a blown voltage regulator so I replaced my alternator and my battery why does my voltage meter only read 9 – 10 volts and it's still drops lower and lower and lower now my car is having a hard time starting please help me. Upon starting my car after replacing all these components the voltage meter never read above 9 or 10 volts and It hesitated to start even with the new equipment in the c