You cannot have three states using just one control line.
Three possible solutions:
1. If you do not need speed control of the motor, connect ENABLE to +5V permanently. Let the PIC controls input2. Use software to invert the state of input1 and outputs that on input2. If you want braking, set both imputs to same state. You can leave motor in braking mode as there is no additional current drain.
2. Leaves out inverter and use an extra pin on PIC for rotation direction control. But I guess you would not have to use an inverter if you got spare pin on PIC in the first place.
3. Use some form of external circuit logic to decode the three state of the input1 namely HIGH, LOW and PULSE and translate this into the "H,L", "L,H" and "H,H or L,L" for the L298.