I'm a bit drunk but what significance does 3 BTC have here?
If you mine solo and find a block doesn't it still pay 25 BTC plus transactions? If you mine in a pool then isn't what matters your rate of finding shares/hour? I can't think of any reason why 3 BTC is relevant to anything - it's 100 GH/S mining for Y*X time or 1 GH/S mining for 100*Y*X time : it's exixtence says nothing about what X or Y is.
It's just their first payment. The next one should come in when Slush gets 100 confirmations on it's next block:
http://mining.bitcoin.cz/stats/Unless they stopped mining or the reward is less then their threshold, or if someone was trolling us by pointing their slush account at that address.