Follow along with the video below to see how to install our site as a web app on your home screen.
Note: This feature may not be available in some browsers.
Also some users have reported turning off use displays in separate spaces fixes the issue. This is a Mac setting under dashboard.

I fear you've spent more time providing your system information than Parallels will reading your post...I just installed Parallels Desktop as well and am experiencing the same issue. Below is my system information.
Software Info
Parallels Version 12.0.2 (41353)
macOS Sierra 16A323
Hardware Overview
Processor Name: Intel Core i7
Processor Speed: 3.4 GHz
Number of Processors: 1
Total Number of Cores: 4
L2 Cache (per Core): 256 KB
L3 Cache: 8 MB
Memory: 16 GB
Display Info
Chipset Model: AMD Radeon HD 6970M
Type: GPU
Bus: PCIe
PCIe Lane Width: x16
VRAM (Total): 2048 MB
Vendor: ATI (0x1002)
Device ID: 0x6720
Revision ID: 0x0000
ROM Revision: 113-C2960K-152
EFI Driver Version: 01.00.507
Displays:
iMac:
Display Type: LCD
Resolution: 2560 x 1440
Pixel Depth: 32-Bit Color (ARGB8888)
Main Display: Yes
Mirror: Off
Online: Yes
Automatically Adjust Brightness: Yes
Built-In: Yes
L2046NV:
Resolution: 1050 x 1400 @ 60 Hz
Pixel Depth: 32-Bit Color (ARGB8888)
Mirror: Off
Online: Yes
Rotation: 270
Automatically Adjust Brightness: No
DELL 2007FP:
Resolution: 1200 x 1600 @ 60 Hz
Pixel Depth: 32-Bit Color (ARGB8888)
Mirror: Off
Online: Yes
Rotation: 90
Automatically Adjust Brightness: No
Connection Type: DVI or HDMI
Adapter Firmware Version: 3.35
Initially I had the setting "Displays have separate spaces" unchecked which caused each monitor to have a separate space with the other two monitors being black in Parallels 12. This is how my system was setup for Parallels 10 and there it would allow me to swipe once between spaces with each VM taking up all three monitors. The best work around I have found was to check this setting (after which you need to re-login). By doing this I am now able to use all three monitors for my VM however I now need to swipe each monitor to change its space instead of just doing it once for all three monitors. This is not ideal but is workable until a better solution comes along.