DIAGNOSIS This is almost certainly a settings issue, not a bad panel. The C3 has multiple HDR picture modes, and some of them (like Filmmaker Mode) prioritize accuracy over punchiness, which can seem dim.
FIX First, make sure you're in a proper HDR picture mode. When playing HDR content, go to Settings > All Settings > Picture > Picture Mode. For gaming, use 'Game Optimizer'. For movies, try 'Cinema' or 'Filmmaker Mode'. Then, in the advanced picture settings, ensure 'OLED Pixel Brightness' is set to 100 for HDR (it's locked there in most HDR modes). Also, disable any 'Energy Saving' or 'Auto Power Saving' features in the General settings, as those will aggressively dim the screen.
IF THAT DIDN'T WORK Check your source device settings. On a game console like a PS5, make sure HDR is calibrated correctly in the system settings. On a streaming box, ensure it's outputting in HDR and not forcing SDR. A misconfigured source can send a dim HDR signal.
HEADS UP Don't crank the 'Brightness' setting (which is actually black level) to compensate. Leave it at 50. If you want more pop, adjust the 'Contrast' setting (which is white level). Also, know that in a perfectly dark room, a very bright HDR highlight on an otherwise dark scene is the intended effect—the dim overall look is part of the contrast.