Well, I'm only guessing, but how well is that connection if it is connected? Probably the signal isn't strong enough and/or the internal wifi adapter is crap.
never had this problem on my rpi1 and 2.
ofc not, because neither of them had a internal wifi adapter 
Have you tried with the wifi adapter you used and RPi1 and RPi2 to see if those are working better?
ofcourse the signal isnt the best, but even then the pi should reconnect after a dc?
ofc not, because neither of them had a internal wifi adapter
i just wanted to say with that, that the problem is related to the rpi 3 wifi adapter, not the router or anything else.
ofcourse the signal isnt the best, but even then the pi should reconnect after a dc?
I asked because there must be a reason why the RPi3 disconnects itself. And if the signal isn't good, this could be one reason. So I would try to use one of the other adapters just for a test.
i can test one of my old adapter, but why they shouldnt work on the pi3, when they do on a rpi1 and 2 with libreelec i noticed disconnects while streaming tv aswell. i gonna test this a bit deeper, but maybe the logs are already helpful?
