The Splunk Validated Architectures document is great place to start with these kind of questions. If you truly need full HA between DCs you might need a multi-site cluster. This will allow you to specify how many copies of the data you want to keep in each site (DC). In the SVA, the examples start with 'M'.
That being said doing multi-site cluster with only 3 indexers is overkill so I'd question whether you truly need it. A normal clustered environment might better suit your requirements if you can accept the risk.