I didn't eat for three days except crackers due to pain and nausea due to kidney stone. Could this?

Chances are it wasn't the not eating that caused your levels to drop but the kidney stone itself...the #1 cause of low potassium levels is kidney disease.

A normal blood potassium level is 3.6 to 5.2 millimoles per liter (mmol/L). A very low potassium level (less than 2.5 mmol/L) can be life-threatening and requires urgent medical attention. Your Dr isnt concerned because 3.3 is just barely below the norm so you have nothing to worry about unless your levels continue to drop.