I have the UltraSharp U3818DW monitor with USB Type-C and so far it's working very well for me. My Stationary PC is connected through the DisplayPort and my laptop through USB Type-C.
The KVM functionality is working just fine.
I need to connect two additional laptops (four in total) preferably through USB Type-C as well so I can use the same mouse/keyboard. I am thinking that some sort of hub much exist that allows me to connect all three laptops at once? Is this possible or do I need to buy an external KVM switch for this? Can you recommend a dock or KVM switch?
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It is feasible with three PC's;
A. PC #1 USB Type-C, No USB selection
B. PC #2 HDMI, Select USB 1 (the USB upstream cable is plugged into USB upstream 1)
C. PC #3 DP, Select USB 2 (the USB upstream cable is plugged into USB upstream 2)
* D. PC #4 could connect to HDMI 2 for video only. It will not use the USB devices plugged into the U3818DW USB downstream ports
A. If you plan to use the shortcut key for Input, it only supports two video inputs. You cannot set it for the third or fourth one due to U3818DW only having two shortcut keys. Use the Menu Input Source to switch to all four PC
B. Only one set of mouse and keyboard are needed for three PC. Use the Menu USB Selection to switch
C. Set each PC to sleep after a longer time such as a few hour or Never (the best) depends on how long you want to use the PC's together. In most cases, switching to sleep/hibernate will not display correctly or cause no video due to USB assigned to that system is inactive unless pushing Power Button on the system
If you're asking if you can just run a USB-C cable from the display into some hypothetical hub and then connect all 3 of your USB-C laptops to that, and allow each PC to send video to that display and work with USB devices attached to the display's USB ports, I don't know of any such device on the market right now. Right now it's virtually impossible to find even a basic data-only USB hub that offers multiple USB-C ports, so I seriously doubt anyone has developed a device that offers multiple USB-C "source ports" that accept video and USB from a PC and will then pass it all up via USB-C to your display.
Your only realistic option to my knowledge is a traditional KVM switch. Connect a DisplayPort and traditional USB 3.0 cable between the display and KVM, and then connect DisplayPort and USB cables from each source PC to the KVM. If your source PCs don't have a DisplayPort output, then use a USB-C to DisplayPort cable for video and a separate USB 3.0 cable for data. But even those KVMs are fairly expensive if you want one that supports 4 PCs, especially given the resolution of the display you want to run. Many KVMs even today top out at QHD.
The IOGear GCS1944 would fit the bill. It actually supports up to 4 source PCs, and on the output side it can run dual displays up to 4K 60 Hz each, which means either output would run your U3818W -- but it's quite expensive. However, a friend of mine uses the 2 PC version of that KVM (the IOGear GCS1942) and he's found that it performs flawlessly. He's using it with a Windows desktop PC and then a Thunderbolt 3 dock, the latter of which he uses with either his PC or Mac laptop at any given time. In fact if you don't need to run more than one of your 3 laptop devices at the same time, that "dock plus 2-source KVM" solution might be better for you too.