If you have version 2.x files that you wish to run with v3.0, use the
version2 style option.
If you have a file formatted with version 1.x, first delete the optional
arguments to the bibitem commands. That is, \bibitem[1]{firstref}
would become \bibitem{firstref}, etc. Then use the version2
style option to run the paper. This means a documentstyle line of
\documentstyle[version2,aps]{revtex}