Can not access 3rd licence

Welcome to the Zoom Community, @AnneS.

Your personal Zoom login can only use one License, so I'm confused that YOU are trying to schedule meetings under another License.  Have you assigned this 3rd license to another user on your account?

You probably need to assign your newly purchased license to another user account. See this web page for assistance:

https://support.zoom.us/hc/en-us/articles/115004976063-Assigning-a-License

Follow these steps taken from the article:

1. Sign in to the Zoom web portal at https://zoom.us.
2. Click User Management, then Users.
3. Locate or search for the user who you want to assign a license. Click the Basic (unlicensed) user to whom the License should be added.
4. Click Edit at the end of the user row. This will open a window.
5. Choose the User Type to assign, selecting Licensed.
6. After choosing the User Type, use the drop down to select the license. There should be an indication that you have 1 available user license if you have indeed purchased one or more new licenses. Click the available license.
7. (Optional) Assign add-ons can be included by checking the boxes for the add-on. You will not likely have any add-ons, so you might skip this step.
8. Click Save.

The license is now applied to your other user's account and they can use it immediately.

If this does not fix your issue, please submit a support ticket to our Billing team here and they will be able to assist you.

https://support.zoom.us/hc/en-us/requests/new?ticket_form_id=360000983032