Without additional model details, etc, the easiest path would be to install the same AOS 6 version on the new controller that you are currently running and add it as a local to your current network.
If you wanted or needed (due to the need to go to newer APs) to go to 8.x, you could set up a similar configuration, with one controller as the Mobility Controller Master (MCM), which is similar to the role of your current Master, and the other controllers as locals. One big difference is that the MCM cannot terminate any APs, so it is functioning strictly as a management box. Also, this does not provide you with many of the new features of AOS 8.
If you decided to migrate to AOS 8, you should install a Mobility Master, which is the management platform for AOS 8, which you can do as a VM. You can even install two for redundancy for no additional cost. Then install your 3 controllers as Mobility Controllers (MCs), which essentially function similar to how your locals did. You would now have 3 MCs that can terminate APs, and a pair of MMs for management. If the 3 MCs are local to each other, you could configure a cluster which could provide hitless failover along with live upgrade capabilities. Running AOS 8 with an MM will provide with all of the new features and capabilites that are offered with AOS 8.
I hope this helps,