I am not sure if I am just slow, but I figured out a hack to be able to use Agenda and Things 3 seamlessly.
Make sure your Things 3 preferences are set up to import reminders from a particular list in reminders. I use a list I call inbox. Inbox is my default, so even siri remiders are placed in this list.
On a note that needs follow up or reminders, I add an apple reminder in the note on the specified reminders list that imports into Things.
Then when you import the reminder into Things, the link to the agenda note is there.
It works great for me. Sorry if this is a repeat post.