I can’t answer the question what is stored in the adb backup.
But if it is “just” the three apps plus general phone settings, you probably don’t need to do a backup at all.
Background: I often test different systems on a secondary device and have to set up the same things again and again and again. I have a list of things to do and that only has 18 entries (e.g. only 5 general phone settings where I don’t use the defaults).
That’s because all the real data is stored elsewhere and I only need to log in to my accounts after installing the respective app.
Yes, that is some work but probably still easier than finding out (the hard way) about the adb backup 
And probably even faster than doing the actual adb backup.