Sounds like the LFS person may not completely understand Tanks, parameters, and tank maturity IMO....anyone with experience in reeftanks would never sell you a nem for a 3 week old tank.
Nems are beautiful creatures, but actually one of the Hardest members of the ocean world to keep in an enclosed system. While many members have had success, many more fail by adding to quickly. Balance of parameters, flow, and lighting are the keys to a Nems survival.
Even if your flow and lighting is acceptable, 3 weeks isn't nearly enough time for the parameters to mature and balance.