Hello Geo17!
You don't need an additional app to set this up. I'm here to help you in giving access to your third user.
It'll now be easier since you already set up the multi-user network with your second user. Now, what we need is to map the network to the third computer so that they can gain access.
Please check this link for the detailed steps on how to map a network drive in Windows: How do I map a network drive in Windows?
Also, I added these articles for your reference in setting up a multi-user network for QuickBooks Desktop:
Lastly, if you're having issues accessing the file with multi-user mode, you may install and run the QuickBooks File Doctor to fix it.
Comment again here if you need more clarifications about the multi-user network. We'll help you!
