I would like to set the test environment using 3 MPC5646C boards for verify flexray that has 3 nodes.
Is it possible? If possible, I wonder how to connect 3 boards.
there are several possible bus topologies. You can see that, for example, here:
FlexRay - Topology | Cypress Semiconductor
Or you can read directly the physical layer spec:
If I connect 3 boards using “active star coupler”, is additional evaluation board that mounted active star (e.g TJA1080) needed?
If yes, is it possible to use XPC5646C as active star? Does NXP have the active star board?
we do not have such board. The easiest way is to use passive star bus topology - you don't need additional HW.