This seems strange. I already had up to 4 USB connected boards on a single RPi 3B and never experienced such behavior.
No idea, but it could point to some hardware defects. I’d try to connect a screen to the RPi and monitor the boot process to see where it fails.
In addition, you can check your Linux system logs and dmesg output.
Note that diagnosing this on a Linux system level requires some Linux admin knowledge. You might find some pointer in this discussion: Pi crashed 3 times mid-print, almost same place - #15 by Sineos