E3 to GC is possible only if your country of chargeability is Australia because you would be able to get the GC within 2 years i.e. before you have to go for visa extension.I know of a case where E3 extension was denied due to pending I-140. In this case country of cheargeability was India.If you were born in India then change to H1 and then apply for GC - safest way. If you were born in Aussie or other country with no retrogression, you can also apply and possibly get GC approved within the 2 years. E3 to GC is still a gray area and no attorney can tell you for sure.