The most likely thing that has happened is that, in reassembling your computer, the airport antenna has come loose. That connection is fragile at best, and is easily separated from the connector on the logic board.
I urge you to take it back and tell them either to fix it or replace it. It is only 3 weeks old, and I worry that if they have botched the service, they could have messed up other - similar - connectors.
Sorry this has happened. I have a MBP that has taken all kinds of abuse..
David