What is Primary Cell and Secondary Cell?
Primary Cell: The Cell UE selects during initial establishment (RRC Connection Request/RRC Connection Re-establishment Request) will become the PCell.
PCell always have both Uplink(UL) and Downlink(DL).
Secondary Cell: eNodeB can add / delete Scell(s) using RRC Connection Reconfiguration message. Scell always have DL (While activated) but may or may not have UL.
UE does not required to acquire System Information and decode Paging from SCell.
For Scell SI is passed to UE while adding the Scell.
For details about Event A3 i prefer you to go to this page
The Measurement reporting event A6 is introduced in Career Aggregation (CA) for detecting and reporting better cell than the Scell on the same frequency on which Scell is camped. A6 reporting criteria is defined as Neighbor cell (same freq as Scell) becomes offset better than SCell(see 36.331 184.108.40.206.a).
Difference Between Event A3 and A6: