QB Issue Resolution:
You install the same way you did on the first 2, using the same license and product number. If you need help with multi-user mode, see below.
Resolution for Issue 'I have quickbboks pro 2016 with 5 licences and only using 2. How do I go about installing on a 3rd computer(3rd Licence).' available: Yes (Solved).
Source: Intuit Community forum.
I have quickbboks pro 2016 with 5 licences and only using 2. How do I go about installing on a 3rd computer(3rd Licence).: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.Support for 5 licences
Quickbooks Repair Pro will resolve it efficiently and affordably. Guaranteed resolution of your issue, or you pay nothing!
Error Recovery for 5 licences [Canada]
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Additional Support Options: