Try using switch #2 or #3 as switch #1. Only connect the computers currently connected to switch #1 is it slow?

Are the computers connected to switch #1 slow when switch #2 or #3 is connected? Did disconnect the computers connected to switch #2 and #3? It could be something running on multiple computers, bad network cards, a bad switch or bad cables.

Maybe a surge or lightning hit the network. I had a switch of mine get hit by lightning and it would work for a few days and then only send out bad packets to everything connected.