Apache TinkerPop is still fully supported, there is a specific OrientDB distribution that includes a Gremlin server and all the TP APIs.
The Muti-Model API in v3.0 is a logical step in the direction of making OrientDB API as simple and complete as possible; in previous versions there was no way to manipulate documents and graphs in a consistent manner with a single API, TinkerPop 2.6 did not have natural Document capabilities and the ODatabase API did not have graph concepts, it was definitely not a nice situation for the end-user usability.
If you are connecting to OrientDB directly, I'd suggest to use the Multi-Model API, that is more optimised and complete.
If you need to use TinkerPop API for some reason (eg. because you need compliance to standards) then I'd suggest to connect to the Gremlin server directly, this will guarantee that you are really using it in a standard way