Generally it’s one project, and thus one analysis, per build. Since you have 3 different jobs for three different sub-projects, I’d simply add the analysis to each of the jobs. And make sure you specify a unique sonar.projectKey for each!
Let me clarify, I have a project have three gradle sub modules.
And I have a Github Actions Matrix Job.
The Matrix Job generates three sub-Jobs, each Job run Unit Test for one sub gradle module.
And I only have a project key.