I only get 2.5 threads with the stock bolts that come with TD3s. The threads are actually 3.5 threads deep but I cannot reach this depth due to the insert starting below the layer of titinal.
I asked Prior and their answer is that 2.5 threads is enough. They have NEVER had anyone on TD3s with the stock bolts rip the threads or inserts out.
I know from other discussions on this forum that it's believed 3.5 threads is required, but if Prior says 2.5 threads is enough that's good enough for me. I have never heard of anyone pulling threads, nor do I think Prior would put any of us at risk of having a binding release while riding.