As far I know, the only way is using get_events method, calling it from xxx_calendar.js, and obtaining data from union sql query. See this:
In your case, you will need joining data from different origins … so, your query should be something like this: (syntax and table names probably are wrong …)
SELECT field1, field2 from 'tabEvent' UNION SELECT field1, field2 from 'tabTask' UNION SELECT field1, field2 from 'tabLeave Application'
Other approach is joining data using python code, generating a dict once data is “merged”.