your linksys router will have a public ip. Then your three computer behind it will have a private ip, like 184.108.40.206
Now in order to connect to your ftp site you need to config the linkysys router to forward ports 20 and 21 to your private IP for your ftp server. Once you have done that you ftp to your public ip and it should get forwarded internal.
Also make sure you have active mode set instead of passive. If you have passive.. then you need to forward more ports.