Hello,
as a workaround you  can try to open the ST2 or 3 projects in safe mode.
For me it had worked to just delete one fx .(Slate, Sonnox or other cpu heavy
fx) and the project opened.
 

Thanks for the extra suggestion Pragi, one I will keep in my back pocket for the future.
 
In the meantime....
Matt, today I tried your suggestions and...Success! With one track in particular I had spent ages replacing all the Miroslav instruments with alternatives but still preferred the original Miroslav version. So I opened the original version, did all of the above (in post #8) and was able to produce a finished mix using the VST2 version without any crashes  That idea of saving the Multi also saved a lot of time. I will now try the same thing on all my other ST3/Miroslav projects. 
 
I will mark this thread as solved although I know that it really needs an updated ST3 VST3 to be properly fixed and keep my fingers crossed for an update from IKM in the near future.

Thanks to all those who responded and to Matt another big 

ZincTrumpet
I still have this problem and it's driving me nuts!
 
So, in summary any new project I create (with latest 2017.06 Splat) using SampleTank 3.6.7 VST2 or 3 works fine.
No crashes, all is fine.
 
If I open projects I created 2 or 3 months ago (probably created with version 2017.03 or 04) which use ST3 they crash SPlat after working on them for a few minutes thus any changes are lost. It so happens that I am putting together an album of tracks created over the last few months, many of which used ST3 instruments. So I am now spending ages opening the songs, documenting which ST3 instruments are being used and then replacing them with alternatives which usually also requires re-mixing due to different instrument levels and sounds. It's a royal PITA!

Sorry to hear of your troubles ZT!
 
I have only had ST3 for the past couple of months, and I have created a few small test projects using the VST3 with it in Sonar 2017.04, 05, and 06.  But I have logged only one Sonar crash related to ST3, and that was a while back.  Cannot remember what I was doing, but it was logged in my Windows Error Reports.
 
Interesting that you say the projects in question open fine, but only crash after working with them for a few minutes.
 
What exactly are you "working" with when the crash occurs?  Is it a feature, control, etc., of the DAW or the VST itself?  Or does it just blow up sitting there or while playing back, or when you try to edit or change some parameter?
 
I had another unrelated issue (not ST3) with Sonar recently when I opened an older project with several softsynths inserted.
 
I could hear a strange resonant sizzling sound in various synths in the upper frequency range.  Easily reproducible with the same project, yet when I opened the same presets in a new project, problem gone.  Same Sonar settings.  I wish I better understood what is saved with a project, that is not part of the global settings, that may possibly get corrupted.