First, don't put in an event for three people when you plan on five. The three people who sign up may only /want/ an event for three, and you might not be allocated enough space for five.
Second, you can get a table to play by going to any HQ and /asking/ if they have a free table from x time to x time. Unless it's at a peak time on friday, chances are they /will/ have such a space, and then it will belong to you for that period of time. No muss no fuss!
<edit> Just 'snagging' a table without asking may work but chances are you'll get booted by whichever event is sheduled there...much better to ask at HQ and get the table assigned.
<edit 2> And if you need a couple more players, post here, on the facebook page, or if it's pathfinder, perhaps over there, to round out your party. The pathfinder area themselves may have extra tables (and players) too!