Hello @julieann, thanks the information. You are right, you cannot add memory to a 3900 series Roku.
If a problem persists across a factory reset, it may be a hardware problem.
I don't know what is going on, but here's a guess:
If you have done a factory reset and can't add channels, that's weird. What error message do you get when you try to add a channel? If it is "out of memory" or "not enough memory," even after a factory reset, I would guess that the flash memory chip has failed in some way and is only operating at a fraction of its true capacity. Unfortunately, Roku has no way to show an ordinary users how much memory it thinks it has installed.
If it is under warranty, I'd exchange it. If it is out of warranty, check to see if the credit card you bought it on offers an extended warranty for purchases.
The strange failure modes you are seeing may also indicate a hardware failure. When flash-memory is semi-functional, parts of code will run, others will crash. There is no way for a regular user to diagnose that sort of thing.
My guess is a hardware failure.
As a last resource you an try a hard factory reset using a paperclip and the reset button on the back https://support.roku.com/article/208757008