Final answer:
The pilot should look to their south if advised of 'TRAFFIC 3 O'CLOCK' while heading east.
Step-by-step explanation:
If an ATC radar facility issues an advisory to a pilot flying on a heading of 090° (which is directly east) saying 'TRAFFIC 3 O'CLOCK, 2 MILES, WESTBOUND...', the pilot should look to their south. The use of 'o'clock' positions in aviation assumes the nose of the aircraft (12 o'clock) is the front. So, '3 o'clock' refers to the right side of the aircraft when facing forward. Since the pilot is heading east, the right side is the southerly direction.