Even though T3G is not advertised on the Esri Training site, we manually post your course attendance and Certificate of Completion there along with any other Esri training courses that you have started or completed. Follow these steps to access your Certificate of Completion for T3G 2019 (or 2015-2018).
What to do if T3G 2019 is not listed in My Learning Activity
PS. This blog was originally posted in the T3G Online Institute group for T3G 2019 attendees. I've re-posted it here for longer-term reference.