Hi,
As you can read in the chapter you're referring to only ASSEMBLER COMMANDS are given to read an input pin or (re)set an output pin. They are of no use to me as I'm programming in a ANSI C compiler.

Do you really think that MCU uses C-operands,commands, etc when it executes a program written with C? (=

Now the question: what does datasheet say about "PUR_3: GPIO Pullup Register For Port 3"?
It says:
The MCU may write to this register. If the MCU sets this bit to 1, the pullup resistor is disconnected from the pin. If the MCU clears this bit to 0, the pullup resistor is connected to the pin. The pullup resistor is connected to the VCC power supply.

And at some lines early:
Each GPIO pin has an associated internal pullup resistor. It is strongly recommended that the pullup resistor remain connected to the pin to prevent oscillations in the input buffer. The only exception is if an external source always drives the input.

So now, could you answer: what shoud you write to PUR_3 according your hardware?

P.S. I hope you know how to write into SFR under C-language.

Regards,
Oleg