When an HADR standby database takes over as the primary database in a multiple standby environment, there are a number of important differences from single standby mode.
With HADR, there are two types of takeover: role switch and failover. Role switch, sometimes called graceful takeover or non-forced takeover, can be performed only when the primary is available and it switches the role of primary and standby. Failover, or forced takeover, can be performed when the primary is not available. It is commonly used in primary failure cases to make the standby the new primary. The old primary remains in primary role in a forced takeover. Both types of takeover are supported in multiple standby mode, and any of the standby databases can take over as the primary. A crucial thing to remember, though, is that if a standby is not included in the new primary's target list, it is considered to be orphaned and cannot connect to the new primary.
Just as in single standby mode, if a failover results in any data loss in multiple standby mode (meaning that the new primary does not have all of the data of the old primary), the old and new primary's log streams diverge and the old primary has to be reinitialized. For the other standbys, if a standby received logs from the old primary beyond the diverge point, it has to be reinitialized. Otherwise, it can connect to the new primary and continue log shipping and replay. As a result, it is very important that you check the log positions of all of the standbys and choose the standby with the most data as the failover target. You can query this information using the db2pd command or the MON_GET_HADR table function.