Possibly, but that's a wooley answer because the language is imprecise here (not your fault, the industry in general has messed this one up for you)
Server could mean two things here; a computer or an application server. A computer can run all three of them at the same time fine as long as it has the resources (memory, cpu etc). An application server (like Tomcat) runs other applications, and is used to run the Atlassian apps.
So, by all means, run them on one server. But use three separate application servers.
(Technically, you could run JIRA, Confluence and Bitbucket in one. In theory. In real life - don't. It is not recommended, not supported, and trashes your data in some cases)
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