I am the account owner and I added a user, so I should be able to delete them.
Hi MadelineE,
I don't know if you got resolution on this, but I had the same problem (also on a Mac - but I don't think that is the issue).
SO THE SOLUTION:
Turns out if that user you are trying to delete is listed as an ADMIN, you need to bump their role down to "MEMBER". Once you do that then, the three little dots will appear. The Help article that tries to explain this is so terribly written, I couldn't understand it... It's here under the section titled "Prerequisites..."
Anyways, hope it works for you! Did for me!