You may find it advantageous to develop your system on a regular Arduino board UNO or Pro mini or Nano whatever where you have plenty of pins and serial i/o easier.
Then when indeed you are really only using three pins and have no need for any serial monitor stuff finding errors you can move the code onto the ATtiny85. With any planning this should be reasonably straight ahead.
i have many projects running in the UNO which woukd be fine on an ATtiny if I ever carried through with the original plan. 
a7