Will a ThunderBolt 3 (USB-C?) to ThunderBolt 2 cable allow me to use a MacBook Pro as second monitor?
If you mean Target Display Mode, then no - that was a feature only available to a select few iMac models &amp; now defunct.

See Apple KB - Use your iMac as a display with Target Display Mode for details.
It was essentially available on iMacs between 2009 &amp; 2014.