Hi, 
 
I'm planing to use only one IC on my PCB design for enabling and reading some pins, less than the whole of 16 available GPIOs. The number of used GPIOs is 13, so there will be 3 of them being not used. 
So my question is:
- what about connection and configuration of this 3 GPIOs? Do they need to be connected following a specific way?
- or could they be floating pins; unconnected and not configured as other devices' pins that are not used?   
 
Thanks in advance for your attention, 
regards!

That chip has programmable weak pullup resistors, so you can enable them on the unused pins (or even all pins).
Alternatively, you could set the unused pins to be outputs.