You’ll want to start with removing the hub, and replacing it with a switch. Because you have a hub in the mix you are sending extra traffic to the switches. All traffic between switch 1 and switch 2 is also being sent to switch 3.
Hubs are essentailly, “passive” in nature, they transmit traffic to every port on the hub at the same time whether it needs to be transmitted there or not. Therefore, you increase the likely hood that two packets are going to collide. A switch, on the other hand, actually learns where traffic needs to go and sends it to where it needs to go on a port by port basis.
What error(s) are your users getting when they can’t get the file server open?