One can have a zero ohm resistor pulling down the three-level inputs. There would not be any functional issues using a zero ohm resistor to set the level to low for each of these inputs.
The three-level inputs of CY7B991 can be modeled by a pull-up and pull-down resistor. The internal resistor values for the three-level inputs are approximately a 25-kohm pull-up and 25-kohm pull-down. The internal circuitry holds unconnected inputs to VCC/2. To pull up/down the three-level input you can connect the input pin to VCC/GND through a resistor. The resistor value must be less than 1 kohm to insure the VIHH and VILL meeting the specification. Zero ohm resistor is okay to use pulling up/down the three level inputs