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.
Basically when you mine in a shared pool, you get your portion of the block based on how much work you did compared to everyone else to earn it.
In this case, "someone" had mined 3.xxxx BTC worth on Slush and had it sent to LC's now official mining address.
So if it is LC, then they are mining. If not, someone just did it to make them appear to be mining to bank on a price rise.
We will know more as time goes by.
That is where the 3 btc comes from.
-Ukyo