Sonnet's own compatibility matrix suggests that using three 4k monitors via their dock is possible (with the caveat that you apparently need to use their Thunderbolt 3 to Dual DisplayPort Adapter).
Theoretically that should be possible since 4k60 uses ~12Gbps bandwidth, and hence 36gbps total...
I'll give you the answer, but first, let me say this: Apple, YOU STINK. Sorry, I had to get that off my chest. <End Rant>
ANSWER: To make a 4K60 external display work with a 2018 MacBook Pro, you need to purchase a USB Type-C to DisplayPort cable, you have to plug the cable into the LEFT SIDE...