Is it possible the other two players have already done other instances or reset lockouts on other characters?
As far as I know, the instance lock is account-wide, at least it is in retail. I haven’t been able to find anything that says if it’s different for classic.
I was doing dm north spam runs killing the first boss for my buddy to try to get a piece off him… I’m not sure If I was seeing things but it almost looked like after the 4th attempt when we would try for 5 it would instance lock. Kind of confusing how it works is it really 5 or is it soft locked at 4. This happened to us twice so we always made sure on the last run to do a full run to avoid cheese.