Question I would ask first: do you want to move data from Oracle to MySQL once or via batch process or your choice is to have replication process running between 3 Oracle sources and MySQL as destination.
1) If this is a batch process, then you may want extract data from Oracle into text file and use loaders for MySQL to load data into MYSQL database. You can use for this purpose tool like WisdomForce FastReader
http://www.wisdomforce.com
FastReader by the way will unload Oracle tables into flat text very fast and efficiently, so you won't get system overhead. Also you can extract Oracle data in selective way by selecting which tables, partitions, columns or even rows you willing to extract. FastReader will generate also control files for loading into MySQL for your convenience. So the entire process can be automated and you can schedule script running FastReader every night for instance.
Questions you should ask when you selecting like WisdomForce FastReader:
- how tool is fast and able to extract large Oracle tables into text files
- will it add system overhead
- will it generate loaders that I need for destination database
- will it support platform that I am using
- does it has GUI and flexibility making selective data unload
- price of course :)
2) If you are looking to replication process it would be more complex and also more expensive way. You would need replication in heterogeneous environment. As far as I remember, WisdomForce developed real-time replication that is able to work between Oracle and MySQL for instance. I didn't found it on www.wisdomforce.com, but you may want contact company directly and ask them.
Questions to ask:
- will it work between Oracle as source and MySQL as destination, how fast it is.
- what is system overhead, and will it be able to handle volumes of data that I need
- does it support platform that I am using
- price
- ....
For replication you will need fast data extract and load tool for initial synchronization before start of replication process. Here again you can use WisdomForce FastReader
Hope this will help.