Possible to run MCP2003 on 3.3v?
The MCP2003 uses TTL inputs and Open Drain outputs for communicating with your MCU.

That means that the TXD pin needs a HIGH signal that is between 2v and 5.3v and the RXD pin swings between 0v and whatever voltage you pull it up to with the required external pull-up resistor.

All of those are what an ESP8266 can work with. Of course you still need to provide the minimum 6v supply to the chip for it to work.

All this is shown in the datasheet both in the form of a simple schematic and a table of voltages:



(Green annotations are mine)

R1 is a pullup resistor for the Open Drain output of the \$R_{XD}\$ pin. It should pull up to the logic level of the receiving device (ESP8266; 3.3V).



(Reduced to only show the two logic voltages of interest)