We booked the Third Ring tour as soon as the dates opened up (a couple months or so in advance) and then purchased the entry tickets online and printed the voucher.
On June 26 we arrived an hour before the tour and were able to skip to the front of the line with our documents. The portion of the security line we waited in was short.
We were directed to a ticket desk (I think it was number 9) and there was virtually no line there. The gentleman gave us our tickets and told us where to meet the tour. We were 30 minutes early and no one else was there until the start time. (The others were part of their own tour as well.)
It was a worthwhile tour, great to have the additional historical context, and access to two areas that others didn't have.