from the WPA rules
the bolded part is from me to accentuate the (to me) relevant to this case rules.
since the 3 was not contacted it "fell into the pocket by itself"
if a ball falls into a pocket because of "settling" it is restored
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8.3 Ball Pocketed
A ball is pocketed if it comes to rest in a pocket below the playing surface or enters the ball
return system. A ball near the brink of a pocket partly supported by another ball is considered
pocketed if removal of the supporting ball would cause the ball to fall into the pocket.
If a ball stops near the edge of a pocket, and remains apparently motionless for five seconds, it
is not considered pocketed if it later falls into the pocket by itself. See
1.7 Balls Settling for
other details. During that five second period, the referee should ensure that no other shot is
taken.
.7 Balls Settling
A ball may settle slightly after it appears to have stopped, possibly due to slight imperfections
in the ball or the table. Unless this causes a ball to fall into a pocket, it is considered a normal
hazard of play, and the ball will not be moved back.
If a ball falls into a pocket as the result of
such settling, it is restored as closely as possible to its original position. If a settling ball falls
into a pocket during or just prior to a shot, and this has an effect on the shot, the referee will
restore the position and the shot will be replayed. The shooter is not penalized for shooting
while a ball is settling. See also
8.3 Ball Pocketed.