On Arduino boards where the microcontroller runs at 5 V (e.g. Uno), the 3.3V pin could be considered output only because it is being supplied directly from a 3.3 V voltage regulator (or a 3.3 V output pin on the USB chip in some cases). However, the ESP8266 microcontroller on the D1 Mini is running at 3.3 V so you can also supply it with 3.3 V via the 3.3V pin.