3 Vitas is little overkill... Why not just selling the two boxed units? You didn't opened so maybe the resale value is good. About the Slim model, I really think that the 1 hour of extra battery is a good deal. You should simply go to a store and see if you like the screen.
If yes, then buy it for the battery, but at least sell your 3 OLED ones. But it's a matter of taste. I have a OLED Vita and won't sell it just to get a new model because I'm happy with my model. If I was buying one now, I would go with the LCD one. Actually a PS TV would be a more interesting (and cheaper) gadget. You can play Vita games on a big screen and if you have a PS4 (like me) then you can mirror it to a second TV in the house.