U3824DW, multiple PCs, 3rd device not working

I'm "upgrading" from the U3818DW where I had 3 devices working using the built-in KVM. Great! Now that I have upgraded, I cannot get the 3rd device to work over the KVM. Why not? This seems like a huge downgrade to me, and makes me regret getting the new version. Outside of buying 3rd party KVM, how can I get the 3rd device working?