Make sure your tuner antenna is correctly positioned for best signal and you can check this by looking at the signal status in TVH while moving your antenna around.
Maybe your tuner is beginning to fail. It does happen. You could try connecting your tuner to a different computer to test it.
If your tuner does not have its own power supply it may be overloading the RPI power supply. In an SSH session with your tuner connected run the command dmesg to see if it reports an under voltage error. If so you will need to replace your Pi3 power supply with a larger capacity one.