DIAGNOSIS A three-blink red light is the TV's way of telling you it detected a serious internal power fault and shut down to protect itself. This is almost always a power supply board issue.
FIX Unplug the TV from the wall outlet for at least 5 minutes. While it's unplugged, also disconnect any external devices (soundbars, game consoles, streaming sticks) from the TV's HDMI ports and USB ports. Plug only the TV power cord back in and try to turn it on. If the three-blink error is gone, reconnect your devices one at a time to see if one is causing a short.
IF THAT DIDN'T WORK The power supply board likely has a failed component. The fix requires a replacement board. You'll need to contact LG support for service or a qualified TV repair technician. This isn't a user-serviceable part.
HEADS UP Do not keep trying to turn it on if the three-blink error persists. Repeated attempts can damage the more expensive main board or panel. The problem is isolated to the power system.