Greetings to you @jennifer_holt !
And welcome to the Canvas Community!
This is a great question!
You may have already viewed the Canvas Support Guide, but in case you haven't: users must be in the specific course for them to be invited to its conference. Thinking out side the box, however, do you happen to know if your three sections are cross-listed together?
The reason why I ask is because, sections that are cross-listed are, for lack of a better term 'merged' so that management of the sections--with the very same content--can be simplified for instructors (students still see themselves as enrolled in their unique sections). As an extra-credit reward for your inquiry that information is here
You've inspired me to run an experiment in the Test Environment, to see if this will work! Thank you!
But let's get back to your issue. You mentioned using ZOOM, and the limitations you are experiencing. The symptoms you are experiencing, lead me to believe you do not have a ZOOM Pro Account. You may want to check with whoever oversee these, at your institution, to see if you may receive a Pro Account. If so, this will remedy the time-limit and user cap.
In the meantime, I'll do some additional due-diligence on this, to see if there is a work-around (via the experiment!).
Of course, the community will come to our aid, such as the incomparable and beautific stefaniesanders, and provide a more direct--if not useful--answer to your inquiry.
Again, WELCOME! I'll reconnect as I find out more information!
Until then have a SUPER Day!
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