I have three computers currently connected to a LAN that also has internet access and is shared by 10 other computers (directly connected to the LANS switches,routers and base stations.), I would like to be able to share folders and file between my 3 computers WITHOUT the others on the wireless and LAN being able to access/see/o find my shared folder.
Easiest solution is to install personal firewalls on your machines and only allow traffic from the computers you wish to be allowed to talk. This works fine assuming you're actually using switches (most likely). Old school "hubs" would broad cast the traffic to all ports meaning someone could "sniff" the raw data off the line. Switches are far more secure as they only send (non-broadcast) traffic to the hardware addresses which actually talking to each other.
You mention however "routers" (plural)... assuming that's the case then you could just put the 3 PC's on their own separate segment and use acl's on the routers / firewalls to prevent the communication.
In short, if you're not sure, just use the personal firewall solution, it'll work fine for all but the most paranoid.