Log Shipping vs Data Replication in SQL Server 2012
If you are using SQL Server 2012 as reporting, then you can use logshipping and configure your secondary as standby - that will allow users to read data from secondary when logs are not being restored. You can delay the restore of log files on secondary as per your needs.

Note that from lower to higher version, you are doing a one way. You wont be able to restore the sql server 2012 databases to sql server 2005 or 2008R2 easily (if you failed over).

Database transactional replication wont be supported as your subscriber (reporting server) would be on a higher version than publisher (sql server 2005 and 2008R2). See my answer here.

Also refer to SQL Server Many-to-One replication