I need to get a small surveillance pc that can support 3 cameras that can store a all the video from the cameras.
zoneminder is a linux cctv solution. They might be worth a look.
http://www.zoneminder.com/
I have never used it but it seems like a cheap way to run a small security camera system on an old laptop or desktop.
I would recommend mini-pc build and connect it with some cheap seagates/wd drives
No need for a pc really, a raspberry pi system would suffice - something like this > http://www.instructables.com/id/Raspberry-Pi-as-low-cost-HD-surveillance-camera/
... but there are several other ultra low cost setups. Or you could store files even on your NAS etc. All really depends on your budget.