I've come across some issues in the past and I would like to submit them for review. However, I've Google'd etc and can't find the right place to do this - anyone?
You should address bug reports to P+T. This can either be done directly from the dashboard of your Control Panel (Get Help widget) or you write an email to firstname.lastname@example.org.
Another way is to create an issue in the public Craft 3 Github repo. It's worth searching there to make sure there isn't an existing issue already. This is a good way to let others know about your issues or to get more of a discussion started about it. I have yet seen anything posted in there getting no attention from the P+T team.
You could always link to an existing question that you started here, that later turned out being rather a bug/issue than sth. the community can help you with ;)
There is a question about how to handle questions concerning bug reports on the Craft Meta Stack Exchange site.
If it's a bug report, then the Get Help widget would be the best route, as it allows you to include your log files and other information.
If it's more of a suggestion, then email@example.com would be your best bet.