The most important factors to consider when looking for a mobile editing station is the speed of its CPU and the amount of RAM it has. Although not crucial, it is better to have a quad-core CPU over a dual-core. Many editing suites take advantage of the extra threads afforded by a quad-core CPU, which will allow you to greatly reduce your rendering times.
I believe the Dell Inspiron 15 i5558-5718SLV to be the best choice of the three you submitted. Of the three, it's the only laptop with a resolution of 1920 x 1080 (1080p). The Asus and Toshiba laptops have a lower resolution, which is less than ideal for editing 1080p video.
That being said, is $500 a hard limit for your budget? There are some great options for a bit more, especially at the $600 to $700 range. Have you considered purchasing a refurbished model? There are tons of great deals out there for refurbished laptops with the added benefit of extra power. For example I found this refurbished Dell workstation on Newegg:
http/www.newegg.com/Product/Product.aspx?Item=9SIA77G3R64732
It has a far beefier processor, and a dedicated GPU. Another option would be this refurbished Lenovo:
http/www.newegg.com/Product/Product.aspx?Item=9SIA3TX39N6551
It does have a dual-core CPU, but the processor is much faster than the new Dell laptop you found on Amazon. It also comes with a 256GB SSD, which should greatly reduce load times.
Ultimately $500 is a pretty steep budget and you will have to sacrifice certain features to reach this price point.