I've recently plumbed my GS/3 and am playing with pre-infusing. My questions are:
What pressure are you using?
How long do you pre-infuse for, and does it depend on your dose?
When using a bottomless portafilter, what, if anything, are you looking for before turning the pump on?

3 bar.
5-6 seconds.
I don't vary the dose.
Nothing until the pump starts.

Thanks for the input Marshall.

I had the sames questions. Thanks for the post.

I wanted to ask these same questions in the setting of not being plugged in:

If the GS3 is not plugged in and the pre-infusion pressure is provided by the steam boiler. Here is the actual quote from the review:

"If the GS/3 MP is plumbed in, preinfusion pressure is provided by the inlet source, either mains pressure or a bottle pump like the Shurflo Flojet. Once the paddle passes the midway point, the pump engages and the brew pressure quickly rises to the pump's bypass setpoint (e.g., 9 bar). In my experience, up to 6 seconds of preinfusion noticeably decreases the likelihood of channeling. If the GS/3 MP isn't plumbed in, preinfusion pressure is provided by the steam boiler. That's sufficient pressure to wet the puck, but I noticed the forgiveness factor suffered compared to when the MP was plumbed in and supplied with 3 bars of preinfusion pressure. As a matter of convenience, the reservoir located beneath the machine is welcome, but to fully realize the GS/3 MP's espresso potential, I would want it plumbed in."

There are other posts that go to great depths in their explanation of why the pressure should be 3 bar. My questions are: how many bars can I get with steam boiler pressure if using reservoir. Is this enough to provide adequate pre-infusion? Is it worth it to attempt pre-infusion if not plugged in? Can you compensate less pressure with more time for the pre-infusion?

Thanks!

Before I plumbed in, when I would try to get a longer pre-infusion from the boiler pressure, I'd get an odd gurgling sound in the boiler after about 3 second.

I run the exact same settings as Marshal on my A3, and yes I have set it up for mains preinfusion. 3 bar input pressure for 5 seconds and I have no drops showing on the basket but the flow starts immediately when the pump kicks in. At 6 seconds I will see coffee starting to peek out of the basket holes. Dose varies but is usually in the 18-19gram range in a LM precision triple basket.