This may sound like a silly question but it has a reason.
When you tried adding your 3rd video did you try waiting 60 or so seconds before editing it?
I’ll explain why I am asking - I haven’t used shadowplay so a little ignorance comes in here on my part but I presume that it is recording game audio as well.
I’ve noted that shotcut take a little time to display the audio waveforms as it moves from one end of a cut to the other - this even occurs on my mighty beast of a machine, give it a minute or two and see if the audio wave form has filled in at the bottom of the clip you added and if this improves your editing stability it could just be that you are trying to do to much to fast and your PC can’t keep up and that does make it look like shotcut crashed but it may still be working in the background.
I notice it way more on my 2600K than on my 6900K.
Hope this helps.
Additional Shadowplay from the little bit of research that I have done records VFR Variable Frame Rate - so convert to edit friendly or running it through some system that converts it to FFR Fixed Fram Rate is required.