Hi NeptuneJ:,
The reason for starting any of these oral antiviral therapies is that someone with HBV infection has reached a certain level of disease to warrant treatment according to current guidelines for the use of these medications. This usually means that there are signs that liver disease is progressing, usually detected by elevated levels of the transaminases ALT and AST in your blood (there can also be other factors that guide a physician to start these therapies). The goal of initiating any of these therapies is to reverse the liver dysfunction (normalize transaminases) and prevent any further progression of liver disease.
In this regards, as Thomas mentioned, there is very little to distinguish ETV, TDF and TAF from each other in terms of how well they work to achieve this goal however here is some additional information on each:
ETV (Baraclude)
Drug resistance is very rare except in patients who have developed resistance to an older oral antiviral called lamivudine.
TDF (Viread)
Tenofovir disoproxil fumarate. Also active against the HIV and widely used in combination treatment for HIV and HIV / HBV infections. HBV drug resistance is exceeding rare, even in patients who have developed resistance to older oral antivirals. ETV and TDF are comparable in antiviral response and long term prognosis (preventing the development of fibrosis, cirrhosis and hepatocellular carcinoma). In the general population the incidence of mild bone demineralization and kidney dysfunction is slightly elevated compared to ETV. As Joan said, this can be problematic in patients more at risk for these issues.
TAF (Vemlidy)
Tenofovir alafenamide. A 2nd generation prodrug of tenofovir (TDF is also a prodrug). TAF has a greater residence time in the liver and as such can achieve similar therapeutic levels of tenofovir in the liver with a lower dose of drug than TDF. So 25mg of TAF achieves the same liver concentration of tenofovir as 300mg TDF.
There are no differences in the antiviral responses between TDF and TAF but with TAF, slightly more people normalized their transaminases (45% with TAF vs 36% with TDF). In addition, bone demineralization is signficantly retarded (but not absent) and kidney dysfunction much less prevanent with TAF vs TDF.
For more technical information on TDF vs TAF you can look here.