For starters, I have no Canon experience... so I can't really say much about the Canon D500.
I own a D5000, which I'm happy with, and I've had a few opportunities to play around with a D3100. Both are nice cameras... but between the two, I would opt for the D5000.
You mention that "super-high quality" video is not a big priority for you, and that happens to one of the new selling points of the D3100. The D5000 only records at 720p, while the D3100 records at 1080p. Overall, I'm not exactly impressed with the D5000's video capabilities... the clips just don't look all that impressive, really. It's clear that you're not using a camcorder when you produce videos with a D5000... and although that may be a "no duh" for many, I think some people actually expect HD video comparable to a unit dedicated to video. The D5000 is a photography camera... it's video capabilities are definitely not too refined. I have not had an opportunity to see if the video functionality and quality has been upgraded for the D3100. Both cameras produce their highest quality video at a relatively poor 24fps, by the way.
The D3100 has a larger screen, goes to 12,800 ISO (unlike the D5k's 6,400 ISO max), and gets two extra megapixels.
The D5000 produces slightly better image quality, detects a bit more dynamic range, and gets 1 extra frame per second burst shooting rate over the D3100. It also has the articulated display, a feature which some love and others seems to roll their eyes at in disgust...
At least one thing that I noticed right off the bat when I used a friend's D3100 for a few minutes: it doesn't offer bracketing. Of course, some people use bracketing all the time, while others rarely bother. For me, that one bit of missing functionality from the D3100 is a huge down-side in my book. While I don't remember many more details of the D3100's menus, I can say that the available options and settings seemed to be somewhat sparse in comparison to my D5000. Again, some people that own a D5000 almost never explore the buried menus of settings and functions. I, for one, use them all the time. So that aspect is important to me.
As you can see, there's quite an assortment of advantages and disadvantages for each model which you'll have to sort through to determine what works best for you. But, as has been suggested earlier, make an effort to actually go to a store and hold each camera. You want a piece of equipment with the features you need, but you also want it to feel right while you're shooting.