Plain English Meaning
This is shorthand for 'the center of gravity moved 3 inches toward the tail.' The minus sign means the shift is rearward, and the 3 is how many inches it moved.
Full Definition
A symbol used in weight-shift calculations representing a rearward (aft) shift in the center of gravity (CG) of three inches. The minus sign indicates direction (aft of the original CG location), and the numeral indicates the magnitude of the shift in inches.
Context Anchor
Seen in weight-and-balance calculations when baggage, passengers, or equipment are moved to a different station in the aircraft.
Derivation
CG stands for 'center of gravity.' In weight-shift problems, a minus sign is used by convention to indicate a rearward (aft) shift, while a plus sign indicates a forward shift. The number is simply the distance in inches.
Why Pilots Care
Even a small unaccounted CG shift can push the aircraft outside its approved operating envelope, affecting stability and control.
Analogy
Think of balancing a ruler on your finger. If you slide a small weight along the ruler, the moves. –³CG is the amount that balance point moved.