Would it be possible for us to stop user select Q3, but allowed us to set the Q3 by loading the value from DB if it set at other place? Or is there any better idea to achieve this? Basically Q3 is one of the option, but is not available to select from this screen
in this case try combo.$setValue()
be aware that onChange will not be triggered.
if you need to process onChange (usually required in form) then use some flags to check if value is set by user. https://snippet.webix.com/s2tlpiac