USB-C to Displayport cable or adapter I'm using MacBook pro 2017 15 inch and i just got 3x DELL U2518D UltraSharp monitors to connect to my macbook pro.

What I actually need is best possible solution to connect this external monitors to my macbook pro. Should i use USB-C to DisplayPort adapter or should i go with USB-C to DisplayPort Cable? 

What is difference? Is cable better? Or adapter?
Use a Type C DisplayPort cable.

The ports on your MacBook Pro are Thunderbolt 3 which is a multiplexing of multiple signals:


PCIe
USB 3.1
DispayPort
Power


Since it natively supports DisplayPort, you want a cable, not an adapter.

Difference between cable and adapter


A cable is just an interconnect between a source and an end device using the same signaling
An adapter creates a signal from one to another (from USB to DVI or HDMI)


In this case, a cable is "better" because it's not converting any signals.  Less conversion means better reliability.