Two of them are under admin
Admin -> system info, and Admin -> user management
The third one should be pretty much on the first page of your my.Atlassian account (I've not got a Cloud install at the moment, so I'm not familiar with the current details)
If they're no longer with you, there's not a lot you can do unless you can get access to their email account. If that's not possible, then you'll need to contact Atlassian support to get your account added ans an owner of the system. Https://support.atlassian.com will help you with that.
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