What is the meaning of this schematic symbol (labelled COMP.J1 JPAD3)
It's known as solder bridge.
A solder bridge consists of two, closely-placed SMD pads that can be shorted easily with solder.
The one shown in the question is a 3-terminal solder bridge. Think of it as an SPDT switch. PDR_ON signal is the COMMON terminal. It can be shorted to either +3.3V or GND.
NOTE: Your EDA software (probably Altium) may not have a solder bridge component or equivalent. You may need to build your own.