Using a KVM switch to connect 3 computers at once

Running two computers at once. 

It happened sort of by accident but now I'm runing my laptop and my
desktop all the time.  And I have a spare computer I really should set
up.  

To use just one mouse and one keyboard and switch them from PC to PC, I
need a KVM switch, but it seems that those that supply more than 2
computers all have HDMI outputs.   And maybe I should get HDMI just
because it's newer than USB. But my laptop and desktop inputs are all
USB.   I gather someone like me is supposed to get an HDMI to USB cable,
but that seems to connect only the mouse OR the keyboard, not both?   So
what am I to do? 

I don't want to use only one monitor. It's good to be able to look at
both so easily .