Chalice is there because with FOW we have 7 cards that matter against fast combo G1 (which could be seen as "enough" protection for a G1 or at least not completely naked should you face a nasty T1/T2 combo deck).
Against fair decks chalice is just a mean to slow down the opponent's game's pace (At least enough for us to be able to build a lethal board).
It can also be seen as virtual cards' advantage in a deck where we don't pack any draw spells.
The main issue with Slivers is that we are (like Merfolk) a slow aggro deck. It takes time to build a board.
Blanking on average 2 cards in an opponent's hand is virtually 2 cards advantage / 2 more turns to kill your opponent.
If you take a sample hand of Slivers, you don't get to the lethal point until T6 /T7. Chalice gives us at least 2 more turns (and straight kill our opponent sometimes; that's what you get for playing unfair, niark)
I ran 3 at that tournament, yes. I was copycatting. It was the first time I had even tried Chalices in the deck, honestly. But now I'm questioning the number. My list in the primer contains 4 Chalices, fwiw.