I have 3 pc connected to a local network with a 4port router
I want to connect an external hard disk to this network but i don't want to use any of computers (because they're not always in use and sometime I use only one of theme)
is there any way to buy a device that share a sata port to this local network
What you are looking for is called an NAS (network attached storage) case for your HD. If you don't own the HD yet, you can for example buy the "Western Digital My Book Live" (the "Live" part is the important one). Otherwise there are cases available in which you can place existing HDs.
These cases do not "share the SATA port", though. Instead they will make the HD available as a network drive.