If you are paying for a multi-user license, then yes, you can access the same data file by three different users, simultaneously. Three users is the maximum for this product, even with a multi-use license.
But if you have just purchased a single license, then no, you cannot use it on three different computers at the same time. If one person on one computer exits the program, then another person on another computer could access the same data file, and so on, one at time. I suspect the EULA specifically states one user, one machine. But there are situations where one user has had to access the same file from several different computers in an office, or sometimes from a remote location. Read the EULA and let your conscience be your guide. :)
I noticed company only provides the outright license of QBD CA Pro edition for up to 3 users and Premier edition for up to 4 users. Should you need having up to 6 users, consider purchasing QBD Enterprise edition for 5 users or 10 users. Another option, you may combine QBD Premier edition with a 3rd party app (e.g inventory management). You may purchase an outright license of QBD 2020 CA Premier 4 users < CA$ 3200.