DIAGNOSIS: That three-blink code from the red LED is the TV's way of telling you it failed its internal power-on self-test. It's almost always a problem with the main board, not the power supply or backlights.
FIX: Unplug the TV from the wall outlet. Wait a full 60 seconds—seriously, time it. While it's unplugged, hold the physical power button on the TV (if it has one) or the joystick button on the back for 30 seconds to drain any residual charge. Plug it back in and try turning it on. If the three-blink code is gone, you just forced a hard reset. If it comes back, the main board likely needs replacement.
IF THAT DIDN'T WORK: If the reset didn't stop the blinking, try a different power cable and outlet to rule that out, but it's a long shot. The persistent three-blink pattern points to a main board failure, which is a known failure point on this series. You'll need to contact TCL support for a board replacement if it's under warranty.
HEADS UP: Don't bother trying a firmware update via USB when it's in this state—the TV can't boot far enough to install it. The blinking code is a hardware fault flag.