We had a coffee gala a few weeks ago and some of my barista friends came over. Of course, the geekdom was supreme! We measured the shots with the timers on our iPhones and even used an iPhone as a flashlight to watch the stripes coming out in the espresso.
After all was cleaned up, I studied the display a bit. The ":" blinks on the clock. I got out my trusty iPhone. Sure enough, it blinks every second. So, now I just count 6 blinks of the ":" to time my preinfusion. Interestingly, I can also hear the preinfusion start and end. There is a very soft whistle as the water fills up the basket. When the 6 seconds are up, or thereabouts, the whistle stops and I assume the basket is filled with water. Then I start the pump. And then enjoy!!

Are you running off the tank or are you plumbed in? If plumbed in then what is the line pressure at the gauge on the panel of the GS/3?

I run 2.5 Bars of line pressure plumbed in.