First of all, Assura 3.1 is no longer supported. The currently supported version is 4.1, so you really should use that. That's the only version available on http://downloads.cadence.com .
Secondly, it's not really a matter of Assura 3.1 being "capable" for 65nm. What matters is whether the foundry you're using has qualified it for their specific 65nm rules. But I really would suggest you install 4.1. There really is no good reason to use an old version.