"The yard has said it can increase and build 3 Virginia's a year. "
Well of course they have - but CAN they ? over the last 30 years getting vessels of any type out of any yard anywhere on time and cost has been a bit fraught. And all the N sub makers have had serious issues.
The fastest way for the RAN is to takeover some early Virginias on lease -that will minimise impact on the USN and allow the RAN to get up to speed with the very different operating environment of an SSN