It depends on the games. Honestly, you'll probably get around $30 or so for all three games, but not much, since the market value of them isn't that high. Fight Night Round 3 is a Platinum Hit, Oblivion has been relegated to the same status with the GOTY edition and Rainbow Six: Vegas dropped in price significantly a couple of months ago.
The price that goes into the credit factors in several things: how much GameStop can make off of it, how much stock GameStop has of the game, demand for the game and whether or not GameStop wants to force the market value down on a product or game. You'll notice you'd only get $30 for a used PS2, despite it selling for $90, simply because GameStop has so much inventory of used PS2s and demand is not high. It's trying to drive the market value down.
Another good example - Lost Planet and Lair were $60 games (I think Lair still is), but their trade-in value was a fraction of that simply due to the overwhelming amount of trade ins.
You don't need any documentation, just make sure they're in good working condition, bring 'em to your local GameStop and get your credit. You'd get 10% more if you have the Edge card (which they will try to sell you on) and an additional 10% if you put it towards any pre-orders.
I will say, right now, if you get a new DS Lite, you get 20% off up to 3 used GBA/DS titles, as well.
