
I've been trying to connect three monitors to Mac and PC for ages and finally found a solution that worked. Connect the supplied cable from the computer to the KVM. Connect the other HDMI cables that came with the kit to the monitors.

CKL Quad Monitor HDMI KVM Switch 4 Port 4K@30Hz with Cables - Extended and Mirrored Display Support for Windows, Linux, and Mac
11 Review

TESmart 1080P 60Hz USB VGA KVM Extender Over Cat5e Cat6 Ethernet Cable - Long Range 984ft/300m (Sender+Receiver)
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NEWCARE HDMI KVM Switch: 4K@60Hz, 2x1 HDMI2.0 Ports + 3X USB KVM Ports - Share 2 Computers on UHD Monitor, with Wireless Keyboard and Mouse Support - USB Disk, Printer, USB Camera Compatible
10 Review

π HDMI 2.0 4 Port KVM Switch with 4K@60Hz, 2K@144Hz,1080P@120Hz Support, 18Gbps Bandwidth, USB 2.0 Keyboard/Mouse/Printer/Monitor/PC Selector, Audio Out/Mic In, HDR10, Dolby Vision, Dolby/DTS, HDCP2.2 Compatibility
10 Review