[Solved] all of a sudden, waaaaaay unstable

What is the nature of the crash? Is it a freeze, the program quits, it sends out a dump report, or...? What OS are you using, 32- or 64-bit? The reason why I ask is that while you may not have made any changes, if you have Windows updates or driver updates done automatically, something could have changed. 
 
You really shouldn't be having constant crashes, I can't remember the last time I had a crash with Platinum so if it happens with Platinum and X3, that might help point to whatever is causing the problem.
 
Also FWIW when something goes bad "all of a sudden" I get very nervous that it might be a hardware issue.

I found X3 and win10 to not be very happy together but havn't had any issues with Splat crashing other than the known issues with Melodyne, but your not using it...
 
Win10 updates are known for 'changing things'. check all your preferences for both PC and SONAR.

Win10 pushed an update yesterday - I got home an my Win10 laptop had rebooted and indicated an update was installed.

As a person with outdated hardware (see my signature, with some luck I get 2 GB moor soon ), I can confirm that running/stopping Cakewalk products "too fast" can cause problems (had that with X1-3, SPlat and RP). But OP is about crashes when X3 is running.
 
The usual sequence: disable all plug-ins in project, increase ASIO buffers, remove all Control Surfaces plug-ins (especially in case you use Mackie/VS-700). As the last resort, switch to WDM mode on build-in Audio. Once you found not crashing setup, start adding disabled features one by one to identify faulty component.

bojackey
I'm on Windows 10 Pro 64 bit.

The think I absolutely despise about Win10 is the forced updates.  I have now had 4 occasions where forced Windows updates have wrecked a computer or cause apps to fail.  Fortunately for me, none of these have been on the machine running Sonar, so this may be completely irrelevant to your situation.
 
The most recent problem happened in December and my computer would just lock up periodically.  I could run applications that were already launched, but I could not launch any new ones, and I could not do an orderly shutdown to reboot.  I did roll back the updates twice, but in a few days, Windows would just install that garbage again.
 
Then there was a package of updates installed a couple of days ago.  Now it seems like the system is stable again.
 
This is probably not your problem, as it appears you have a more specific issue.  But as a general bit of guidance, if one has Win 10 Pro version, it is possible to instruct Windows to delay any non-emergency patches.  Highly recommended.