I use a KVM to connect 2 PCs to 1 set of peripherals: monitor, keyboard & mouse. This is standard KVM usage, & it works well for me.
You'd like to connect 3 PCs to 2 monitors but controlled by only ONE keyboard+mouse - is this correct? If yes, I'd be surprised if a KVM or similar device exists that will allow you to accomplish this.
However, if you use TWO sets of keyboards+mice, you could connect it in the following way:
PC1 (the dual-booting one perhaps) connected directly to 1 set of peripherals (including the 1st monitor)
PC2 & PC3 connected to a standard KVM using the other set of peripherals (including the 2nd monitor)
Of course, a greater than 2-port KVM will allow you to connect all 3 PCs to one set of peripherals, but that would leave out the 2nd monitor. G