Does it matter if you are the only one administrating wordpress? and for the wiki i guess it depends on if you want to keep it closed or open (editable) for everyone. If you want the wiki to be open for all..., then it's a bit tough. I think that if you keep it editable for only a few people that they don't really mind. This also saves a shitload of time preventing bots to register and write their ads in the wiki.
so basically only a few people will need the login for 3 systems. Every other visitor just needs a forum login.