you should always make a reservation for 8 people, no matter the restaurant. in fact, as most restos need to pull two tables together for a group of 5, i'd suggest calling for anything over 4 people even if the restaurant accepts walk-ins.
also: why on earth are you going to les 3 brasseurs?
Agreed - this place, for whatever reason (I guess location), often seems to be really busy, particularly on Friday nights so it might be best to reserve a day or two ahead of time.
But I also agree with the above post regarding the choice of restaurant. I don't mean to sound pretentious about it, but chowhound is a place where good food is discussed so I think its our duty to try to recommend the best restaurant options for your criteria. With this in mind, I dont think there are any criteria that would merit going to 3 brasseurs. Their food is as bad as their beer, and I honestly think it lacks any redeeming features other than a central location. Especially if you're coming from out of town I'd consider trying some other place - even in the terrible downtown vicinity there is better food to be had at around the same price range.