Should I get a used laptop, xbox 360 slim or a ps3? My budget is around $300?

Consoles are just very specific computers. If you really need something, laptop might be your best choice as it can be upgraded with external devices later on and can be used for more than just gaming (ie work processing for work/school, music/video editing, etc) and is more mobile.
If your budget is really that tight, then you might want to hold off on buying something until you can really afford something. Also, used laptops might have issues such as a virus, worn out drives, reduced battery life, etc. Sites such as newegg tend to have brand new laptops for low prices.
Another pro for a laptop is all the free to play games out there these days.