How to access macOS localhost (port 3000) from Parallels IE?
One of the way I could use it, I found my local IP4 address provided by the router. 

From Mac system preferences &gt; Network &gt; IP Address: 192.168.x.xx

And let say if you access it in Mac with http://localhost:3000 you can do it also with http://192.168.x.xx:3000 from Mac itself and from Parallel Windows OS with shared network configuration.

This IP address could be changed each time you restart the router or choose another router etc.

Currently, I'm using Parallels Desktop 12