Can standby database of a physical database be pluggable or vice versa?
Can standby database of a non-container database be pluggable or vice versa?


In order to plugin a non-CDB standby database into a CDB, the non-CDB standby database needs to be of version 12c as well. So existing non-CDB databases will need to be upgraded to 12c before they can be part of a 12c CDB. 


  Suppose there are 2 CDBs on 2 different server. And one has 3 primary PDBs, the other has 3 standby PDBs of the primary with dataguard configuration. If the switchover executed on one primary, will it execute switchover on other 2 PDBs?


The dataguard switchover or failover operations are all performed at the CDB level. When the Data Guard is configured with the CDB database, all PDBs are also replicated to the target. Same way when a failover or switchover operation is performed all the PDBs also change role along with the CDB container.

As migration is a serious task I would recommend to take look at Oracle Support Knowledge based documents.(MOS account required)
Step by Step Examples of Migrating non-CDBs and PDBs Using ASM for File Storage (Doc ID 1576755.1)