Have a project called "spiral ham" (....read my Catanya post. Exact same result.)
 
This project has 3 reaktor spirals and 3 raptures.   Used to crash Sonar within 3 seconds, now spiral was running for 200 bars not a hiccup.  I changed all kinds of things on the fly...stopped, started, never an issue.
 
Did the same experiment and recorded midi to three separate midi tracks simultaneously, all worked flawlessly.
 
I will continue to work with Sonar X3d.  If I see any thing I will let you all know, but so far so good.
 
I am over the moon...thanks cake.
 
 

I have tried my experimental setup that used to crash immediately.
I used 3 Reaktor Spirals running 3 separate VSTs.
In this particular case I used Chromaphone, Alchemy and ZebraHZ.
 
The project ran for quite some time, several minutes.
 
But as I was tweaking the various VST settings, Sonar X3d froze at 6:30 minutes.
 
Far better than the instantaneous crash, but still not quite right.
 
 EDIT - typo Reaktor froze -> Sonar X3d froze
 

That is why right now in all my test projects I use cakewalk synths only, and I am being very careful to point this out.  (Dim pro, rapture, RGC, etc.)  Once I get comfortable with cakewalk included Vstis i will venture to Komplete....   Unfortunately I do not own any of the vstis you mention above so  could not re create.
 
 

auto_da_fe
That is why right now in all my test projects I use cakewalk synths only, and I am being very careful to point this out.  (Dim pro, rapture, RGC, etc.)  Once I get comfortable with cakewalk included Vstis i will venture to Komplete....   Unfortunately I do not own any of the vstis you mention above so  could not re create.

Oh no I wasn't suggesting you use the plugins I recommended. I figured your test was already 95% set up to reproduce the issue and all that was required was just another MIDI track recording MIDI from an external keyboard. But no worries, Shawke has confirmed that this issue is apparently no longer reproducible so I'm going with that.

I tried to reproduce this crash with a new project and it looks like I was wrong.
I had 3 Reaktor instances of Spiral driving 3 VSTs and it ran fine.
Also using other VSTs like Kirnu Cream I encountered no problems.