SOLUTION:
Yes, I have found the solution myself.
In order for me to explain how to stop what happens, I must explain why it happens.
Basically, the “parse error 3” is a JSON parse error when there is an issue reading some text or transferring a text file.
This generally only occurs on things like Word, Excel, etc. so why did it happen on FiveM? This is due to a rare bug when FiveM doesn’t read your Steam account name correctly and has invalid (according to FiveM) credentials.
One of the ways to attempt to get around this is by pressing F8 and typing in ‘connect’ and then the IP address directly.
However, this will not work in my case so I had to delve further.
Of course, Steam always has to be open (even when the accounts are linked) in order for FiveM to make sure you have GTA5.
I think this is one of the main problems I was having. I did have Steam open, that wasn’t the issue, but it still refused to understand my account. I had also set a “Connection Name” which is what is seen as your username while connecting to the server. This is ok to do, but it can cause problems.
So the issue in short is that FiveM and Steam do not communicate well together, and you need to have a way to completely refresh both your FiveM and Steam. Once Steam refreshes, FiveM follows (as long as the accounts are linked).
Make sure you open Steam FIRST and foremost before opening FiveM.
So how does this get fixed? How do you refresh Steam and FiveM together?
The answer is to change your username on Steam.
Why does this work? No idea. But it makes FiveM have to relog your account information, which is good and it will realize, oh, you do have GTA.
Another problem you may encounter while using the F8 method is seeing that you cannot connect to a certain IP address. This is because the IP is being blocked by your antivirus. You don’t need to disable the IP blocking, but it can help as well.
Well, it’s late and I hated fixing this. If all else fails idk what to tell you, but if anyone finds this useful in the future in this rare bug you are welcome.
I talked about this extensively with many mods and admins from FiveM, literally nobody has ever gotten a solution to this. I’m very happy to finally be able to play with my friends.
(TL;DR) Change your username, press F8 and then type “connect [the ip address for the server]”, avoid special connection names, reload everything