It's just been pointed out to me that the devices I previously recommended do not support 3 x 4Kp60, but do 2x 4Kp60 using the DisplayLink chip and 1x 4Kp30 using alt-mode.
However, following your original set up - you can search for adapters using this link:
https://www.displaylink.com/products...840x2160&num=1
Kind regards,
Andrew