First you need to wait for Personas 3 to finish ramp-up and become generally available. That's currently expected by the end of Q2, but the date isn't finalised yet.
Second, you will need a kernel upgrade. Personas 3 needs the 7.42 kernel, and the most recent patch level of that. Your Basis version is already new enough.
Then Personas 3 installs as an ABAP add-on, as Personas 2 did. They co-exist in the same system - installing 3 does not remove or disrupt 2 in any way. There will likely be a list of OSS notes to apply after the install, unless you wait for the first support pack - date unknown but likely in early Q3.
Finally, there's an automated migration process to convert your version 2 flavours to version 3. Again the conversion does not disturb the originals, so you can run both for a while to avoid a big-bang switch for your users, if you wish.
Steve.