I've tested my USB on other laptops and computer and it works just fine.

However, when I try to boot to live USB using my main computer it boots to the screen where I select "Start KDE neon..." and then I just get a black screen, no input. I booted both in normal and UEFI BIOS mode, still the same thing? My main computer also has 3 monitors one of them being a 4K one, not sure if that matters but this live USB works on all my other computers/ laptops.

Any idea in what I can do to get it working? I am using KDE neon user edition (non LTS)

Press <e>dit in boot-menu.

Add 'nosplash' to the 'linux'-line of the BootEntry.
Also, additionally try

'plymouth.enable=no'
'systemd.confirm_spawn=yes'


You'll see the booting process's separate steps.

Report back the last step (where it doesn't proceed).
Before it gets black.
What's the last message You see?

Seems to only happen when all of my 3 monitors are connect as I tried it with just the one and it worked, I plugged the other two in while neon was running and everything went **** up from that point, screen froze and nothing worked. I tried what you said but the message on the screen went to quick for me to read it. I have my monitors plugged into my Nvidia GPU, a 1080 if that helps?

Unfortunately, I don't have
a multi-monitor setup available
for testing.

But:
Additional info from Your side would certainly help:

1)
Are You using the default nouveau driver or
are You using nVidia's proprietary driver?

2)
Where does Your 'freezing' happen?

Before the graphics-Desktop started
or after start of X Window System?

(sddm-Login? plasma-Desktop?)
(Immediately? After a while?)

3)
If You are still trying to install:

Install with one monitor only.

Use default nouveau drivers only,
at first.

Update linux-kernel to newest available
HWE (HardWareEnablement) version.

Activate additional monitors one by one
in plasma's settings.

4)
If nothing works, and You install
nVidia's proprietary driver:

Boot into TEXT-mode (systemd.unit=multi-user.target)!
Don't install the proprietary drivers, while X Window System is running!