Q3 is a low-cost PCH FET and is used to keep CHG away from any voltages below VSS. When CHG is not being pulled high, PACK± being pulled below VSS will not be seen by CHG as Q2 does not turn on. Q3 also allows R2 to keep Q1 OFF, since all voltages below this FET can Q3 "follow" PACK± as it goes below VSS.