Snagit 3 is not supported on macOS Sierra. If 3.3.3 is working, I would stick with that until you're able to upgrade to 4. (You can turn off the autoupdates in Snagit preferences to make sure it doesn't accidentally happen)
Upgrading to 4 doesn't guarantee anything. I have 4.0.5 and This morning I hit a stray key not knowing which and it crashed, the very first crash I have ever had with SnagIt. SnagIt has been the most reliable application i have ever used. I just updated to Sierra Night before last took 9 hours to download and install and 95 Mbps Comcast cable connection.
I suggest the People at Techsmith better get going. Oh and don't believe that 35 minutes to download and use nonsense floating around on the internet.