Best Buy's standard return window is 15 days from purchase, so you're unfortunately a week past that cutoff. The good news is that dead pixels are usually covered under manufacturer warranty, even after the retailer's return period closes. Samsung typically considers a pixel "dead" if it's completely black and unresponsive, and having five of them should qualify for a warranty claim or replacement under their defect policy.
Your best move is to contact Samsung directly through their support line or website - bring your receipt and take a photo or video of those dead pixels so you can show them. Be specific about the location and how many there are. They often authorize replacements or repairs without much hassle if it's within the first year. You could also call Best Buy's customer service to ask if they'll make an exception given that three weeks is still pretty early, though don't expect much there once you're outside their window. Worst case, you've got a TV that works fine with a few dead pixels; best case, Samsung sends you a replacement unit or arranges service.