How to treat a row as a table
You need to use multiple self joins to get that information:

select c.personelid, 
       a.username as accounting, 
       f.username as finance,
       h.username as hr
from confirmations c 
  left join users a on c.accounting = a.userid
  left join users f on c.finance = f.userid
  left join users h on c.hr = f.userid;