Connect 3 monitors to Macbook Pro (2018) using one cable?
You can totally do this. Pick any AMD GPU that meets your needs - there are several that drive 3 to 6 displays in the $120 to $200 price point and put that in a thunderbolt 3 EGPU case.

https://www.amazon.com/dp/B0745GHN53/ref=cm_sw_r_cp_tai_DT.yFbXV1S9X3

Pretty much anything you can put in a PCI slot will work over thunderbolt and one cable. Basically you’re putting a GPU for the new Mac Pro like the Radeon Pro W5500X MPX Module in an external enclosure.

https://store.apple.com/xc/product/MXM52AM/A