Welcome to JUST ANSWER heavy equipment. With years of experience, I look forward to doing my best to help.
Center pedal is for auxiliary valve if so equipped. Used for clamp attachment or auger or similar attachment You may or may not have the third valve, just the pedal.
Usually if you have the third valve it has pipes running to the front with disconnects or plugged off. Or you can look at the control valve for the third spool.
If you have joystick, the middle pedal is likely not attached to anything. Check to see if the linkage is even there.
How many auxiliary hoses do you have out front? Two or four?
So you only have three spool valve, You are already using the auxiliary for the clamp/
yes, are you using the auxiliary connections on the clamp?