Conditional Inner Join?

Im faced with the following design issue..

on my site there are different profiles: a city profile, a restaurant profile and a user profile.

in my DB:
City profiles are stored in tbCities
 cityID int PK
 shortname nvarchar(50)
 forumID int FK
 (...)

Restaurant profiles are stored in tbRests
 restID int PK
 shortname nvarchar(50)
 forumID int FK
 (...)

User profiles are stored in tbUsers
 userID int PK
 shortname nvarchar(50)
 forumID int FK
 (...)

as you can see a single ID value (for CityID,restID or userid) might occur in multiple tables (e.g. ID 12 may exist in tbRests and in tbUsers)
Each of these profile owners can start a forum on their profile.

forumID in each of the above tables is a FK to the PK in tbForums:
forumID  int
forumname nvarchar(50) 
(...)

Now imagine the following:

a site visitor searches ALL forums...say he finds the following forums:
ForumID Forumname
1 you opinion on politics
2 is there life in space?
3 who should be the next president of the USA?

a user may want to click on the forum name to go to the profile the forum belongs to.
And then there's a problem, because I dont know in which table I should look for the forum ID...
OR I would have to scan all tables (tbCities,tbRests and tbUsers) for that specific forumid,which is time-consuming and I dont want that!

so if a user would click on forumID 2 (is there life in space?)

I want to do a conditional inner join for the tablecontainingforumID (which may be tbCities,tbRests or tbUsers)

select tablecontainingforumID.shortname FROM tablecontainingforumID t
INNER JOIN tbForums f ON t.ForumID=f.ForumID
where f.ForumID=2


I hope my problem is clear..any suggestions are welcome (im even willing to change my DB design if that would increase effectivity)