What parts of SPLAT are missing from CBB, that aren't already included in X3 Producer?
 
Are there plugins and 3rd party features in Platinum that aren't currently in CBB that weren't already a part of X3 Producer?
 
Thanks!

There must be 50 threads on this topic.
 
There's a sticky right at the top here that has the compairison chart, they put it there so people would stop asking this question. 

Forget about the plugins bundle for a minute just for the sake of discussion.  The last version of Sonar, Platinum 2017-10, has almost three additional years of new core DAW features and bug fixes developed since Sonar X3e.  It is a much better program.
 
You get the latest core Platinum DAW features for free included in CbB. Plus any plugins you have working in X3 Producer should be recognized by CbB and function same as always.
 
Cakewalk Sonar, what's new, is available here.  Pull up a comfy chair, this is going to take while to get through. 
https://www.cakewalk.com/.../SONAR/Whats-New#start

X3 had terrible problems with the editing of tracks/takes and envelopes. These all get corrected in Platinum. 
 
X3 track editing is only a glimmer of what Platinum track editing is.
 
X3 track editing = unfinished and buggy.
Sonar Platinum track editing = finished and polished.
 
X3 had orphaned envelope problems and the idea of takes were still in their infancy while recording new takes Sonar X3 picked whatever take it wanted to record in or over and you could not designate which take a new recording would go to.
 
Editing tracks/envelopes/takes is a breeze in Platinum/CbB.
 
Piano roll is much more polished too.

You will be able to see the Sonar Platinum Features in the spreadsheet linked in the sticky thread above. That will give you an idea of what is newly added to Sonar since your X3e Version.