ESP8266 pin D3 question

I connected a switch to ESP8266’s pin D3 with a 4.7K pull down resistor, and pin D3 reads high when the switch is open and when it’s close. When I use pin D0 it reads low when the switch is open as it should. I wonder why D3 is odd.
The ESP8266 diagram says GPIO0 and Flash for pin D3, by the way.

D3 is not suitable for input

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